4 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



iii 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 in situ, I quote the 

 following from the First Annual Report of the Geological Survey of Texas, 

 by Professor Eobert T. Hill (pp. 124, 125) : " These fliut 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, 

 so 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. The work herein described was confined 

 largely to three nodules, which were prepared in the following way. 

 Thin sections were cut and mounted in Cauada 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 he detected, if present. The sections were 

 CHt 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 

 winch it is my pleasure to acknowledge. 



