4 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
in thickness, cemented with infiltration of silica. Generally, there is but 
one layer of chalky substance, but in one nodule I found four, alternat- 
ing with layers of solid, amorphous silica giving the whole the ringed 
appearance of a concretion of sand or clay. 
Of the occurrence and appearance of these nodules n situ, I quote the 
following from the First Annual Report of the Geological Survey of Texas, 
by Professor Robert Т. Hill (pp. 124, 125) : “ These flint nodules are found 
in the Caprina chalk and chalky limestone subdivision of the Comanche 
Series of the Cretaceous of Texas. . . . They are oval and kidney-shaped, 
ranging in size from that of a walnut to about two feet in diameter. 
Exteriorly they are chalky white, resembling in general character the 
flint nodules of the English chalk cliffs. Interiorly they are of various 
shades of color from light opalescent to black, sometimes showing a 
banded structure. ... The fact that these are the only flint horizons, 
80 far, at least, as is known to the writer, in the whole of the immense 
cretaceous deposits of the United States, is very interesting, and espe- 
cially since they occur about the middle of the Lower Cretaceous Series, 
instead of the top of the Upper Series as in England." 
The nodules of Comanche County, according to Professor Hill, contain a 
nucleus, Monopleura Texana, around which the flint has formed, but in 
the nodules that I examined the silica was solid throughout and there 
were no nuclei of any kind. Тһе work herein described was confined 
largely to three nodules, which were prepared in the following way. 
Thin sections were cut and mounted in Canada balsam, just as rock 
Slides are made for petrographical study. These sections were made at 
the circumference and near to the centre, perpendicular to the surface 
and approximately parallel to it. In addition to these, several slides 
were made at random in each nodule. This precaution was observed in 
order that the difference in preservation of organic remains between the 
interior and surface might be detected, if present. Тһе sections were 
cut at different angles to detect if there be a tendency toward definite 
arrangement of organic remains in the nodule, — a tendency which might 
result from the constant application of any external force unchanging 
for even a short period of time. Caustic potash and acids were tried in 
identifying the mineral material of the replacement, but on account of 
the very hard nature of the mass of the nodule, it was found best to use 
the polarizer only, which produced more satisfactory results. During 
the progress of this work, I have been the recipient of favors from Dr. 
Robert T. Jackson and the Boston Society of Natural History, both of 
which it is my pleasure to acknowledge. 
