THE PETROLOGY OF SEDIMENTARY ROCKS Ves 
somewhat different application of these principles. The writer in 
the course of an examination of a portion of the ceded lands of the 
Crow Indian Reservation, Montana, during the past summer, 
followed the outcrop of a coal bed in the Lance formation for a 
distance of about thirty miles; and throughout this distance the bed 
contains a parting of material which closely resembles a brown 
carbonaceous sandstone. This parting varies only from three- 
fourths of an inch to 14 inches in thickness, and its constancy and 
persistence proved a decided aid in recognizing and correlating the 
coal bed. Such characters are exceptional in the Tertiary and 
Cretaceous continental deposits of the west, and the writer was 
prompted therefore to examine microscopically several specimens 
of the parting. In thin section this material proved to be, not a 
clastic sandstone as supposed, but to be a nearly pure aggregate of 
delicate mineral crystals, which undoubtedly formed in situ. The 
optical characters correspond to those of Leverrierite.' The 
mineral is now being more carefully examined and analyzed and the 
writer hopes to discuss it in a later paper. 
It is apparent of course in this instance that the practical value 
of the small layer did not depend on microscopic examination, since 
use was made of it in correlation before its real nature was known. 
Microscopic study in this case merely served to explain its per- 
sistency and homegeneity, the characters which rendered it of value 
in the field, and to throw light on its origin and its relation to the 
coal bed and other strata. Moreover, the formation of such a pure 
mineral deposit in the midst of a heterogeneous mass of largely con- 
tinental sediments is certainly of scientific interest; and if it is, as 
the writer is inclined to believe, not an uncommon development, 
but merely one which lack of previous petrologic work has held us 
in ignorance of, its bearing on the accumulation and alteration of 
sedimentary rocks is manifestly important. 
Practical difficulties —The microscopic study of sedimentary 
rocks is attended by certain practical difficulties which the writer 
can attempt only to suggest. In the first place the fineness of the 
material and the extent of its decomposition are important obstacles 
to the study of shale. Where the grain is not too uniformly small, 
t First fully described by P. Termier, Ann. d. Mines, XVII (1890), 272. 
