THE PETROLOGY OF SEDIMENTARY ROCKS 7285 
to inquire very closely into the proportions of its component 
minerals or the shape of their grains. Many sedimentary rocks, it is 
true, have been examined and described as lithologic types for 
educational purposes," or because of some peculiarity in distribution 
or occurrence,’ and there-are a few instances in which they have 
been carefully studied for economic reasons,? but little work of a 
genetic character, directed upon the rock as part of a mass or 
formation rather than as a more or less fortuitous rock type, has 
been done. 
THE IMPORTANT LINES OF RESEARCH ON SEDIMENTARY ROCKS 
During the past decade attention has been directed to sedimen- 
tary rocks by the masterly studies of Johannes Walther in Germany 
and the amplification of his work in this country by Barrell, Grabau, 
and others. These geologists have, however, dealt almost entirely 
with the stratigraphic relations of the beds, and such structural 
features as cross-bedding, mud cracks, etc. In endeavoring to 
ascertain the manner of deposition of the rock, whether for example 
as a deep-sea deposit or a river sediment, a delta or playa lake, they 
have assigned somewhat minor importance to its petrology, con- 
sidering chiefly its broader megascopic characters. Considerable 
work has also been done upon the transportation of sedimentary 
material by water and by wind‘, and the consideration of criteria by 
which the amount of such movement may be estimated. Thus W. 
H. Scherzer’ has constructed a classification of sand grains based 
largely on their size, their degree of angularity and the amount of 
1 Such descriptions are given by Diller, Bull. U.S. Geol. Survey No. 150, 1898; by 
Harker, Petrology for Students; by Kemp, Handbook of Rocks; by Hatch and Rastall, 
Petrology of the Sedimentary Rocks; and by the authors of most of the other textbooks 
on lithology. 
2 See for example L. Cayeux, ‘‘Structure et classification des Gres et Quartzites,”’ 
Congres geol. internat. C.R., toth Session, 1906, pp. 1211-22. 
3 See for example, C. B. Berkey, Bull. New York State Mus., No. 146, 1911, pp. 
124-48. 
4See for a summary, with very complete lists of references, E. E. Free, ‘‘The 
Movement of Soil Material by the Wind,” Bull. Bureau of Soils, U.S. Dept. Agric., No. 
68, IgIl. 
5 “Recognition and Classification of Sand Grains,” Bull. Geol. Soc. Amer. (XX1), 
Igo, 625-62. 
