464 THE INSCRIPTIONS AT COPAN. 
grains, generally rounded in shape. Many of the primary quartz grains are cor- 
roded and embayed. Inclusions of long, needle- shaped crystals of high refringence 
were observed in several of the quartz sections. 
A brown weathering crust was observed on one of the specimens. The junc- 
tion between this crust and the adjacent interior is sharply defined; the weathered 
shell is more or less banded; but even in this shell the plagioclase feldspar still 
preserves its clear, glassy character. 
Under the microscope some of the feldspar sections are more or less regular in 
shape, but many are irregular and corroded. They are embedded in a much altered 
matrix which is not easy to decipher in all its details. In each thin section angular, 
irregular, short lines of dark, dusty material appear in the field; these are commonly 
curved and resemble the fracture lines of an original tuff rich in glass and pumice 
fragments. ‘These are now profoundly altered and the interstitial spaces are more 
or less filled with secondary material, much of which is submicroscopic in size. 
Some secondary quartz is present in fine veinlets and aggregates; also calcite in thin, 
thread-like lines. Chlorite is common and fills cracks and interstitial spaces; in 
some cases it appears to be an alteration product of original hornblende, but the 
evidence is too indefinite to be decisive. Small grains of iron oxide surrounded by 
a brown alteration zone are common; they are probably titaniferous magnetite. 
In the ground-mass, much of the material 1s too fine for satisfactory identifica- 
tion. ‘There is present some argillaceous material, some secondary calcite, some 
quartz; aggregates of radial spherulites of a colorless, weakly birefracting mineral 
of positive elongation and refractive index about 1.480 occur, especially near cor- 
roded plagioclase feldspars. In the ground-mass there is present an isotropic or 
weakly birefracting substance of refractive index about 1.460. 
In view of the profound alteration of the matrix, the relatively unaltered con- 
dition of the plagioclase crystals is surprising. It appears that the glassy tuff frag- 
ments suffered rapid devitrification and subsequent alteration, whereas the primary 
plagioclase and quartz crystals and fragments remained unchanged, except for mar- 
ginal attack and alteration. 
The evidence, so far as it can be gathered from the few small specimens, points 
definitely toward their tuffaceous origin. The conditions which led to induration 
of the fragments of tuff which now appear as inclusions in the larger masses of rock 
can be ascertained only by field work. Both the inclusions and their host may 
properly be designated andesite tuffs; the presence of quartz in some of the speci- 
mens and the lack of ferromagnesian minerals indicates that the rocks are high in 
silica and alkalies and approach dacite tuffs in composition. 
The causes which led to the profound alteration of these rocks and even the 
details of the changes as presented in the few specimens at hand can not be profit- 
ably discussed without further field evidence. 
GEOPHYSICAL LABORATORY, 
CARNEGIE INSTITUTION OF WASHINGTON. 
