abstracts: geology 25 



Included in the paper are tables showing the occurrence and geo- 

 logic range of Upper Cretaceous fossils, a table indicating graphically 

 lithologic variations and age relations of the Cretaceous deposits, and 

 a geologic map on a scale of 1:2,500,000. 



The value of the genus Exogyra in correlation rests on the fact of 

 its extended geographic distribution and the restricted stratigraphic 

 range of its principal species, Exogyra costata and Exogyra ponderosa. 



L. W. S. 



GEOLOGY. — Slate in the United States. T. Nelson Dale and others. 

 U. S. Geological Survey Bulletin No. 586. Pp. 220. 1914.i 

 Slate, in ordinary usage, denotes a rock with more or less perfect 

 cleavage adapting it to various commercial uses, and in which the 

 particles with few exceptions can not be distinguished except in thin 

 section under a microscope. Slates are classified, according to genesis, 

 into (1) sedimentary and (2) igneous; group (1) being by far the most 

 important and subdivided into: 



A. Clay slates: Matrix without any or with very faint aggregate 

 polarization. 



B. Mica slates: Matrix with marked aggregate polarization. 



A further subdivision of each group is based upon mineral character. 

 Igneous slates are divided into (A) ash slates and (B) dike slates. 



The slates of group (A) originate in marine deposits of clay and 

 sand. The angular grains of feldspar and quartz in slate imply the 

 nearness of masses of granitic rocks. The alternation of slate beds with 

 quartzite or grit corresponds to the alternation of extremely fine clayey 

 with sandy sediments from such land masses. These sediments con- 

 sisted largely of quartz, feldspar and mica, with zircon and other sili- 

 cates, various compounds of iron, lime and magnesia, together with kao- 

 lin from the decomposition of the feldspar. 



The next stage in the formation of slate is attributed ultimately to 

 the radiation of the earth's internal heat and the consequent corru- 

 gation of its outer portion. One effect of this compression was to 

 metamorphose the shale into slate. This included two processes of 

 uncertain priority. Under the combined presence of moisture and the 

 effect of pressure and heat, both the heat generated by the pressure and 

 that which pervaded the strata at the depth at which they were buried, 



1 This is a revised edition of Bulletin 275 of 1906. The coworkers include: W 

 F. Hillebrand (analyses), E. C. Eckel, A. F. Purdue, L. M. Prindle, F. H. 

 Moffit, (S. A. Bonine (geology), M. Merriman (tests), Miss A. T. Coons (statistics). 



