APPENDIX I. 
A PETROGRAPHIC DESCRIPTION OF THE MATERIAL OF THE COPAN 
MONUMENTS. 
Frep. FE. Wricur. 
Small specimens from three monuments, Stele D and 3 and Altar U, were 
examined and found to be essentially the same in general appearance and type. 
In each one of these rock-masses, small inclusions of a denser, harder rock occur 
which differ from the host only in the fact that they are more indurated and contain 
more quartz. ‘The rocks as a group are so highly altered that their original char- 
acters are difficult to decipher with certainty. They are evidently of tuffaceous 
origin and range in composition from dacites to andesites high in silica. The lack 
of ferromagnesian minerals, such as amphiboles, pyroxenes, and micas, is note- 
worthy. In the ground-mass, dusty areas, more or less rhombic in outline, may 
be seen here and there and may represent former amphiboles; but no definite proof 
was obtained of the presence of any one of the ferromagnesian minerals in the 
original rock-mass. 
In general appearance the specimens are dull, porous, fine-grained rocks, pale 
green and yellow-green 1 in color, mottled here and there with colored angular areas, 
such as abound in altered tuffaceous rocks. The clastic texture is more clearly 
marked in the dark-colored inclusions than in the inclosing rock-mass. At first 
glance these inclusions appear to be fresh, vitreous rocks containing phenocrysts 
of clear, glassy plagioclase and quartz; but on closer inspection with a magnifying 
glass, and especially in the thin section under the microscope, they are seen to be 
clastic in nature and not essentially different from the host, except for the greater 
abundance of secondary quartz. In these indurated fragments the sharply curved 
intersections and interstitial spaces between the glass fragments of the original tuff 
are well preserved and give to the rock an unusual and characteristic appearance. 
Many of the angular and rounded cavities are lined with a white to pale green crust 
of soft material which under the microscope is cryptocrystalline to microcrystal- 
line, and weakly birefracting, with an average refringence of about 1.535. Many 
of the cavities contain, in addition, secondary quartz. The spherical shape of some 
of the cavities, I to 2 mm. in diameter, suggests bubbles in an original glassy lava. 
In the inclusions these gashes and other cavities are common and demonstrate 
the tuffaceous origin of the rock. In the less indurated rocks the cavities are still 
present, but they are not so apparent in the hand specimen because of the generally 
altered, dull condition of the samples, which has obliterated all contrasts. 
Scattered through each specimen are clear, glassy crystals of plagioclase feld- 
spar measuring up to 3 mm. in diameter. These crystals are developed as stocky 
prisms elongated along the axis oo1:010; tabular development after the side pina- 
coid (o10) is less common. ‘The forms feeaned on the crystals are: (oor), (010), 
(110), (110), 101). Cleavage after oo1 is well developed. The feldspar sections 
show slight zonal structure and average in composition an albite-oligoclase of the 
composition AbsAn;. Fine albite twinning lamella are common; Karlsbad twin- 
ning lamella were observed in a few of the feldspar sections. Many of the feldspar 
crystals are irregular in shape and are evidently fragments of fractured crystals. 
Quartz grains are less abundant than the feldspars. They occur in water-clear 
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