Petrographic report on rock samples from the Banyano River Dam Site, Panama

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Petrographic report on rock samples from the Banyano River Dam Site,

Panama

PETROGRAPHIC REPORT ON ROCK SAMPLES FROM THE BANYANO RIVER DAM SITE, PANAMA

by

P. E. Grattan-Bellew and E. G. Swenson

Internal Report No. 407

of the

Division of Building Research

OTTAWA July 1973

by

P. E. Grattan-Bellew and W. G. Swenson

PREFACE

Several samples of volcanic rock from the Banyano River in Panama were sent to the Division of Building Research by Montreal Engineering Limited, with the request that they be tested for

potential alkali- expansivity. Although the samples were not of Canadian origin it was decided to respond to this request from a Canadian company which in turn, would extend this Division's list of tested rock types. Such volcanic rocks are not common

III Canada but they are found in some localities.

The reaction between some rocks and the alkalis in cement can cause deterioration of concrete. This problem may be particularly severe in large exposed structures, such as dams and bridges. As it is usually impossible to stop the reaction once started

expensive repairs on a continuing basis are often necessary throughout the life of the structure. For this reason it is essential to avoid

using rocks that may be reactive or to take precautions to prevent or minimize the reaction such as using low-alkali cement or adding a pozzolan to the concrete.

At present the mechanisms of the alkali-reactivity reactions are not fully understood. A research project is currently being carried out in the Division of Building Research which it is hoped will further elucidate the mechanisms of reaction and provide the information required to catalogue the reactive rock types found in Canada.

With current knowledge of the mechanisms of the alkali-reactivity reaction, the potential of rocks to expand can only be determined with reasonable certainty by making length-change measurements on concrete prisms, mortar bars, or rock prisms when applicable. These tests are being made but the results will not be available for some time.

This petrographic report has been prepared to provide preliminary evaluation of the potential reactivity of the rocks. Although a petro-graphic report alone cannot provide positive evidence, combined with previous experience it can give a good indication of the potential of a rock to undergo expansion in cement.

Ottawa July 1973

N. B. Hutcheon Director, DBR/NRC

BANYANO RIVER DAM SITE, PANAMA by

P. E. Grattan-Bellew and E. G. Swenson

At the request of Montreal Engineering Company Limited, some rock samples from Panama were examined and tested to determine their potential for alkali-reactivity in concrete. The samples, which were received in the fall of 1972, are representa-tive of rock types being considered for use as aggregate in the construction of a dam on the Banyano River. A covering letter from Montreal Engineering Company Limited (File MEO 2660-1.

Ba-202) stated that petrographic examination and quick chemical tests, carried out at the University of Panama, indicated the rocks to be potentially reactive; the Division was asked to obtain confirmation of the potential reactivity of the rocks.

The material received can be classified into two rock types: i) volcanic tuff (breccia), and ii) fine-grained volcanic rock (andesite). Of the five core samples from Tigron quarry, four consist of tuff, one of andesite. The four river gravel samples are composed mainly of andesitic lava, with minor rhyolite and basalt.

The potential alkali expansity of a rock can only be determined from petrographic examination when experienc e has already been

gained on identical rock types, and even then some doubt remains; for example, Duncan et all found that some Nova Scotia greywackes and quarzites were reactive, others not. The best that can be achieved from a petrographic examination is some indication on the potential for alkali reactivity of the rock. Gillott et al2 (Table 2, p. 20) include among Potentially Reactive Rocks rhyolites, andesites and volcanic tuffs. So far reactivity has only been demonstrated for rhyolites.

The ASTM C-289-66, Quick Chemical Test for Potential Reactivity of Aggregates, gives another indication about the potential reactivity of a rock, but the results are not always conclusive. For example, tests carried out on Nova Scotia Rocks at the Nova Scotia Technical College and at the Division of Building Research in Ottawa, yielded conflicting results. (Fig. 6, p. 26 1). For some reactive rocks, one laboratory would indicate a sample to be reactive, the other non-reactive, and vice versa for a second sample. From these results it must be concluded that the quick chemical test is not very reliable.

At present, conclusive evidence concerning the potential reactivity of a rock can generally only be obtained from mortar bar tests, ASTM C227 - 69. One case is reported by L. Dolar Mantuarris Ip, 229), in which a mortar bar test showed a feldspathic syenite to be expansive but a concrete darn in

which this rock was used as aggregate was in satisfactory condition after 25 years. One problem with the mortar bar test is that

conclusive results are not obtained until one or two years after the beginning of the test. When it is not possible to wait that long for results, reliance must be placed on the quick chemical and petrographic tests. When these tests indicate a rock to be expansive and in the absence of confirmatory mortar bar tests, it would be prudent to use a low-alkali cement or add a pozzolan to the concrete to prevent or minimize alkali expansion.

Mortar bar tests are currently in progress on the samples from the Banyano River; the results will be the subject of a separate report to be published later.

SUMMAR Y AND CONCLUSIONS Petrographic Examination

1. Drill Core samples from Tigron Quarry

Hole 11, # 1 (55 ft). Tuff of andesitic composition. constituents: Fine-grained volcanic material, calcite, chlorite.

Main feldspar,

Hole 11, #2 (863/4 ft). Volcanic tuff. Similar to

#1.

Hole 11, #3 (97 1/2 ft). Highly altered tuff, broadly similar to # 1 but containing in addition epidote.

Hole 17, #1 (26 1/2 ft}.Highly altered tuff similar to #3. Hole 17, #2 (40 314ft). Similar to 17 #l.

Hole 12, # 1 (18 ₁₁₂ ft). Porphyritic andesite consisting of phenocrysts of feldspar and altered pyroxene in a fine crypto-crystalline matrix.

As the core samples did not provide sufficient material for both the quick chemical and the mortar bar tests, it was decided to eliminate the former.

2. Gravels from the Banyano River Four samples were rec eived: (i) Guayabo Island upstream (ii) " " c e n t r e island (iii) Conchihua Island upstream (iv) " " downstream

A visual examination indicated that all the samples were essentially similar and they are therefore discussed together.

Coarse Aggregate (plus 114-in. fraction) consists mainly of rounded pebbles of volcanic rocks. Andesite is the commonest, with minor amounts of rhyolite and basalt. Occasional chert and igneous rock pebbles were also observed. It was decided that a detailed petrographic examination of these rocks was not warranted.

Fine aggregate (minus 1/4-in. fraction).

This fraction consists of a pale green glassy material with about 5 per cent each of quartz and feldspar.

Quick Chemical Test

The results of the quick chemical test on the gravel samples

indicates that they are non-reactive (see Figure I, which is reproduced from the ASTM Book of Standards and on which the results of the

Rhyolites are known to be reactive. In addition, Gillott et a12 list, among potentially reactive substances, "acid to intermediate silica rich volcanic glasses" occurring in rhyolites, andesites and volcanic tuffs. All three types occur in the Panama rock samples. It is concluded from the results of the visual examination of the gravel samples and the petrographic study of the rock cores that all the samples may show alkali expansion in concrete. The

results of the quick chemical and petrographic tests are conflicting, and confirmation or otherwise of the potential reactivity of the rocks must await the results of the mortar bar tests which are unlikely to provide an answer before the end of 1973, at the earliest. If it is not possible to wait for the results of the rn o r ta r bar tests, it would be safest to assume that the rocks are reactive and take the necessary steps to counteract this. There is probably little likelihood of finding another source of aggregate near the darn site. The Banyano River drains an area compos ed mainly of andesitic lava flows.4 A letter from Montreal

Engineering Company LiInited (7 September 1972, File MEO 2660 BA-202) states that tests carried out on a limestone horizon, overlying the tuff, in the Tigron quarry, indicated it to be potentially reactive. (No details of the tests were given.)

Other sedimentary rocks occur in the lowlands but these would probably be too far from the darn site for their use to be feasible even if they were non-reactive.

PETROGRAPHIC DESCRIPTION OF DRILL CORE SAMPLES FROM TIGRON QUARRY

Some of the details given in the descriptions of the thin sections may seem rather superfluous for the job on hand, but it was believed necessary to provide as complete documentation as possible in the hope that it will be a contribution toward a better understanding of the process of alkali expansivity and thereby to our ability to predict which rock aggregates will cause problems in concrete.

Hole 11, #1 (55 feet) (DBR 72-6CJ)

With a hand lens, the rock is seen to consist of a mass of small fragments, mostly rounded, which range in size from 1 mm downwards.

from dark dusty-green 5G 3/2 to pale green-yellow 5GY 7/4. The rock is non-magnetic, as evidenc ed by its inability to deflect a small suspended magnet. In thin section the rock consists of a fine cryptocrystalline matrix in which can be distinguished small plagioclase feldspar laths, varying from about O. 1 to 0.3 rnrri in

length. In the fine matrix, fragments of other fine grained

volcanics occur, some dark, others light in colour (Fig. 2). Some of the fragments of fine-grained volcanics contain between 10 to 15 per cent of pyrite. The pyrite occurs as small cubes which are opaque in thin section (Fig. 3). The cubes are, on the average, about 100 ｾｭ square. Pyrite and other opaque minerals (iron

oxides) occur scattered throughout the section.

Potassium-feldspar occurs as large euhedral grains up to 0.5 rnrn long. All the feldspar has a brown, cloudy appearance due to alteration. Many areas of chlorite were also observed (Fig. 4).

The chlorite is distinguished by its pale green colour in normal light and by its ultra blue polarization colour. Chlorite is an alteration product of ferromagnesian minerals and glasses of a similar composition.

Calcite occurs as large grains and fine interstitial material. Some of the feldspars have euhedral hexagonal outlines, similar to those in Fig. 15. Hexagonal crystals of this appearance are characteristic of feldspathoids, e. g ,; lucite, and it is possible that the feldspar is pseudomorphous after them.

One rounded vesicle filled with calcite and quartz was observed (Fig. 5). The calcite is readily identified by its rhombohedral cleavages and its characteristic appearance in polarized light. Quartz is characterized by its general appearance and uniaxial interference figure. Occasional small grains of epidote were observed. Approximate mineralogical composition of the rock as determined by visual examination in the microscope was as follows: to 15% to 30% to 2% to 5% to 20% 10 20 1 2 10 1 to 3%

rock is composed of fine cryptocrystalline calcite k-feldspar epidote quartz chlorite opaque sulphides and oxides

The remainder of the volcanic material.

The rock is a tuff, probably approximating to an andesite in

composition. Areally it is associated with the andesite flows which make up most of the rock in the head-waters of the Banyano River

(Ref. 4, p. 25-27). The presence of epidote, chlorite, calcite

and pyrite are typical of rocks altered by a process of propylitization6_•

Hole 11, #3 (97 1/2 feet) (DBR 72 - 62)

This is a fine- grained f r agrn en ta.I rock. With a hand lens its colour is a mottled greyish green (5G 5/2) to off-white (N8).

The rock is non-magnetic.

In thin section the rock is seen to consist of a very fine-grained cryptocrystalline matrix in which only fine plagioclase laths are visible. In the matrix, subrounded to subangular fragments of fine-grained volcanics occur (Fig. 6); also embedded in the matrix are calcite crystals of all shapes and sizes, up to about 2 mm in diameter. Potassium feldspar occurring as altered cloudy grains is common.

Chlorite occurs as large irregularly shaped masses similar to those shown in Fig. 4.

Epidote occurs as large irregular grains (Fig. 7).

Epidote is easily distinguished by its high refractive index and high order polarization colours.

Quartz occurs as small isolated grains and occasionally as fracture infilling.

Some fragments of igneous rock were seen (Fig. 8). The igneous rock occurs in patches showing an ophitic intergrowth of pyroxene and feldspar. (The pyroxene is altered to chlorite.) Such textures are characteristic of some igneous rocks, e. g , , norite.

Numerous small fractures cut through the rock, some filled with calcite others with chlorite and quartz (Fig. 6). Some fractures are confined to individual volcanic fragments indicating that they were fractured either before or during their inclusion in the tuff. Sulphides and iron oxides (pyrite and hematite were identified)

occur scattered as small grains and large patches throughout the sample, they show up black in Fig. 8.

Approximate mineralogical composition: calcite 30 to 400/0 k-feldspar 15 to 250/0 quartz 1 to 20/0 epidote 5 to 80/0 chlorite 1O to 150/0 opaque sulphides and oxides 5 to 70/0.

The remainder of the rock is made .up of fine cryptocrystalline volcanics. The rock is a highly altered tuff. It is difficult to determine its original composition but it was possibly derived from an andesitic volcano.

Hole 17, #1 (26 1/2 feet) (DBR 72 - 63)

This is a fine- grained fragmental rock having subangular -to-rounded particles varying from 5 rnrn diameter downwards. The colour is mottled, due to variation in the colour of the individual fragments: white to dark green. The over -all average colour is pale olive green (10 Y 5/2). The rock is non-magnetic.

In thin section the rock is shown to be highly a Ite r ed, much of the original material having been converted to epidote, quartz and calcite. In the fine matrix occur fragments of volcanics similar to those described in the samples from hole II.

Occasional large potassium feldspar phenocrysts are visible in the matrix; they are about 0.3 rrirn., long and are readily

identified by their twinning and relatively low refractive index. (Fig. 9) A typical area of the section showing a mass of calcite, quartz, volcanics and opaque minerals is shown in Fig. 10. The average size of the quartz grains is about 50 urn, Some parts of the slide consist of about 30% epidote, (Fig. 11) Pyrite and hematite make up about 2% of the rock. Approximate mineralogical composition: calcite 30 to 40% k-feldspar 5% quartz 10 to 15% epidote 15 to 20% chlorite 10 to 15% opaque minerals - - 2%

The remainder, about 20%, is composed of fine-grained volcanic material.

This rock is a tuff broadly similar to those already described but possibly more altered. The presence of fine-grained quartz suggests that this rock might be susceptible to alkali reaction in concrete.

Hole 17, #2 (40 3/4 it) (DBR 72-64)

This is a medium- grained fragmental rock with particles varying from 0.1 to 0.5 mm in diameter. The variations in the

colour of the fragments: dusty green (5G 3/2), pale olive (10 Y 6/2), pale green (5G 6/1) and white, impart a mottled appearance to the rock. The rock is non-magnetic.

In thin section this rock looks somewhat less altered than those previously described. There are large areas of fine crypto-crystalline volcanic material up to 2.5 rrirn in diameter but the average size is about 0.5 rnrri, Twinned plagioclase and

potassium feldspar occur both as large euhedral crystals

(Fig. 12) and smaller fragments. The feldspar has a brownish clouded appearance possibly due to ka ol.iniaation, Epidote is a common constituent of the rock. It occurs as irregularly shaped particles with a high refractive index. The epidote varies in size from 0.05 to 0.3 rrirn , Quartz occurs as small isolated grains or clusters of them in what are probably

vesicle fillings (Fig. 13). The serrated interlocking edges of the quartz grains should be noted. The grains vary between 50 and 200 lin in diameter.

Calcite occurs as occasional small grains and probably also as fine interstitial material.

Approximate mineralogical composition:

of fine-grained 5% 20% 5% 20 to 25% 10 to 15% 5% cent is composed calcite k-feldspar quartz epidote chlorite opaque minerals --The remaining 20 to 40 per volcanics.

The rock is a tuff possibly of andesitic afinities.

Hole 12, #1 (18 1/2 it) (DBR 72-65)

With a hand lens two types of euhedral phenocrysts can be seen in the fine-grained dark green-grey (5G 3/1) matrix: one dark green-blue (5G 2/1) the other whitish. The rock is non-magnetic. Through a microscope the rock is seen to consist of a very

fine matrix in which only fine plagioclase laths are distinguishable (Fig. 14). Four types of phenocrysts are distinguishable in the fine matrix:

1. Potassium feldspars - - some showing twinning. These crystals are altered (Fig. 15), as were those of the samples previously discussed.

pseudomorphs after feldspathoids. A typical feldspathoid would contain about ZO per cent of KZO. This section contains about 5 per cent of these hexagonal crystals, accordingly an analysis should show about

1.

°

showed only 0. 11 per cent. This indicates that either some KZC has migrated away during the alteration of the rock or that the original minerals were not feldspathoids. The former hypothesis is preferred but no evidence can be offered for it.

3. Pyroxene

Pyroxene occurs as twinned crystals (Fig. 14). They can be distinguished from twinned feldspars by their higher re-fractive index and higher order polarization colours.

4. Chlorite occurs as an alteration product of some pyroxenes.

In ordinary light the chlorite is pale green, in polarized light dark blue, which shows up as black in Fig. 14.

Pyrite and iron oxides occur as small grains scattered throughout the section.

The rock is a porphyritic andesite. Approximate composition: k- feldspar ZO to 30% feldspar pseudomorphs - - 5 to 7% pyroxene 1

°

The remainder, about 50 per cent, is composed of fine-grained cryptocrystalline material in which only plagioclase laths are distinguished.

Comments

In all the rocks described, there is a small amount of fine-grained quartz, with individual grains in the 50 to ZOO !-Lm range.

This fine quartz may well be reactive with the alkalis in cement, but as it only comprises about 5 per cent of the rocks it is unlikely to cause a problem. The fine cryptocrystalline matrix of the

rocks is more likely to be reactive; it is little different f'r om volcanic glass which is described by Gillott et a12 as being potentially reactive.

MACROSCOPIC DESCRIPTION OF GRAVEL SAMPLES FROM THE BANYANO RIVER

Sample lA, Guayabo Island (upstream)

A quick visual examination suggests that the dominant material comprising the gravel is volcanic rock of acid to intermediate composition. An assortment of pebbles was broken for closer examination with a hand lens or binocular microscope. Four rock types were identified:

L Fine-grained medium grey andesite containing about 1 per cent of finely disseminated sulphides (pyrite).

2. Fine-grained tuff containing some calcite. With a hand lens this is seen to be a fragmental rock and is probably similar to those described in the thin sections.

3. Weathered, pale dusty yellow 5Y 6/9, very fine-grained rock. Probably a rhyolite.

4. Coarse-grained rock, with a grain size of 1 to 2 rnrn , It is composed of quartz, feldspar and biotite/amphibole. The rock is badly weathered. It probably corresponds to a syenite in composi-tion. Only one pebble of this rock was found in the sample. Acid igneous rocks are extremely rare in central Panama. 4

Sample 2A, Guayabo Island (centre stream)

This sample is similar to sample lA and consists mainly of acid to intermediate volcanics. A number of pebbles were broken for closer examination, 3 rock types were identified:

1) Fine-grained dark blue-grey (5B 4/1) rock containing 1 to 3 per cent of disseminated sulphides. The rock is probably a basalt.

2) Mid-blue grey (4B 4/1) fine-grained rock of andesitic composition. Sample IB, Conchihua Island (upstream)

The rocks making up this sample are similar to those of the Guayabo Island samples. Four rock types were identified.

(i) Medium greenish- grey volcanic tuff, similar in appearance to the andesites, apart from their fragmental texture.

(ii)Medium greenish-grey rock. On fresh surfaces it is seen to consist of laths of dark grey pyroxene or amphibole in a pale grey feldspathic matrix. This rock is coar ser in grain size

To facilitate separated (iii) Pale grey fine-grained rhyolite containing about 1 per cent

of dis s ern in.at.ed sulphides.

(iv) Yellowish white very fine- grained rhyolite, probably a weathered s arnpl e of the grey rhyolite.

SaInple 3 B, Conchihua Island

An initial visual exarn inati on indicates that this s arnpl,e is s irnila r to the s arrip l e lB, Four types of rock were identified.

(i) A rn o tfl ed rn ed iurne g rey to white, m ed ium fine-grained rock,

consisting of a grey feldspathic rna tr ix in which are ernb edd ed laths of pyroxene or arnphib ol e and whitish euhedral crystals (feldspathoids). The rock is either a porphyritic andesite or a trachite.

(ii) A rn ed iurn grey rock s irnil.a r to above but with a rn o r e patchy texture, probably a tr ac hite ,

(iii) Fine-grained volcanic rock with about 1 per cent of finely dis s ern.iria ted sulphides; it is a basalt.

(iv) A rock s irniIa r to (iii) but with a paler colour suggesting a c orrip o sition between basalt and andesite.

(v) Pebble with a very srriooth surface, m ed ium brown in colour. The rock is a fine-grained chert and was probably derived f r orn a vesicle in a lava flow. Only one chert pebble was found in the four s arrip l.e s ,

Sand fraction (Ininus 1/4 in.) of the Banyano River Grav,els The sand f r orn all the s arrrpl.e s is essentially s irni.Ia r , exarn iria ti on with the binocular rn ic r os c op e, the sand was into several size fractions, using standard screens.

-4+ 10 rn e s h: In this fraction f r agrn ent.s of acid to in te r m ed ia te volcanic rocks dorn ina te , There is about 5 per cent each of quartz grains and green glassy rna te r ia l .

-10+14 rn e s h: 10 to 15 per cent of quartz grains, about 25 per cent of pale green glass and 60 per cent rock f r agrn ent s c ornp ris e this size fraction.

-14+ 18 rn e s h: Pale green grains d orn iria te the s arnp l e, There is about 10 to 15 per cent of quartz and about 10 per cent of volcanic rock f r agrn errts ,

-18 rn e s h: Pale green grains rnak e up about 90 per cent of the sarnple , The r erria ind e r is c orripo s ed of quartz and feldspar. The quartz is generally present as angular fr agrn ents but

occasional needle-like crystals were observed. An x-ray powder photograph of the pale gr een grains showed them. to consist of pyroxene and probably s orn e volcanic glass. The presence of the glass is indicated by broad diffuse bands on the x-ray diffractogram.. The sand fraction has a com.position which is consistent with its being derived from. acid to interm.ediate volcanic rocks containing som.e secondary quartz. The fine-grained glassy m.aterial m.ight be expected to be reactive with alkalis.

CONCLUSIONS

All the gravel sam.ples consist dom.inantly of fine-grained acid to interm.ediate volcanic rocks. These rock types are known to be reactive with the alkali in cem.ent and consequently if this gravel is to be used as aggregate either a low alkali cem.ent should be used or a pozzolan should be added to norm.al portland cem.ent.

The core sam.ples from. the Tigron Quarry, with the exception of one porphyritic andesite, consist of volcanic tuffs of acid to interm.ediate character. There are currently no data available on the perform.ance of these rock types as aggregate in concrete, but from. the sim.ilarity of their m.ineralogical com.position to acid and interm.ediate volcanic rocks, rhyolites and andesites, which are considered to be reactive, it is concluded that these rocks should be treated as reactive and the usual precautions should be taken to prevent or m.inim.ize expansion if they are used as aggregate in concrete.

REFERENCES

1. Duncan, M. A. G., E. G. Swenson, J. E. Gillott and R. Foran, Alkali aggregate in Nova Scotia concretes. Atlantic Industrial Research Institute, Nova Scotia Technical College. 1970.

z.

Petrography of Concrete Aggregates and Field Cores Nova Scotia. Atlantic Industrial Research Institute. Scotia Technical College (undated)

in Nova

3. Dolar -Mantuani, L. Alkali-Silica-Reactive Rocks in the Canadian Shield. 24th 1. G. C. Section 13, 1972. p. 227 -234

4. Terry, R. A. A Geological reconnaissance of Panarna ,

Occasional Paper No. XXIII, California Ac ad erny of Sciences, San Francisco. 1956.

5. Rock colour chart. Geol. Soc. America, 1963.

6. Moorhouse, W. W. The study of rocks in thin section. Harper and Harper Geoscience Series, 1959. p. 197.

FIGURE 2. CROSSED POLARS. X25. HOLE 11

#1,

FIGURE 3. HOLE 11

#1,

PYRITE CUBES IN A FINE-GRAINED MATRIX.

ABBREVIA TIONS: LA. VOLCANIC FRAGMENTS F. FELDSPAR. PY. PYRITE.

FIGURE 5. HOLE 11

# 1.

VESICLE FILLED WITH CALCITE AND QUARTZ.

ABBREVIA TIONS: O. OPAQUE MINS. CH. CHLORITE. C. CALCITE Q. QUAR TZ. F. FELDSPAR.

FIGURE 7. HOLE 11

#

ABBREVIA TIONS: CA. CALCITE. V. VEIN, (FRACTURE FILLING. ) LA. FRAGMENTS OF VOLCANICS. EP. EPIDOTE.

T

FIGURE 8. HOLE 11

#

OPHITIC INTERGROWTH OF FELDSPAR AND PYROXENE.

FIGURE 9. HOLE 17

#1.

LARGE CALCITE CRYSTAL IN A MATRIX OF FINE CALCITE AND VOLCANIC MA TERIAL.

FIGURE 11. HOLE 17

#

ABBREVIA TIONS: Q. QUAR TZ. O. OPAQUE. CA. CALCITE. EP. EPIDOTE.

FRAGMENTS OF VOLCANICS IN A FINE MATRIX.

FIGURE 13. HOLE 17

#

CLUSTER OF FINE QUARTZ GRAINS WITH SERRA TED INTERLOCKING EDGES, IN A FINE-GRAINED MA TRIX.

ABBREVIATIONS: F. FIELDSPAR. LA. FINE-GRAINED VOLCANICS. Q. QUAR TZ.

ALTERED FELDSPAR, PSEUDOMORPHS AFTER FELDSPATHOIDS (?) IN A FINE-GRAINED MATRIX.

FIGURE 15. HOLE 12

# 1.