470 STEVENSON— THE FORMATION OF COAL BEDS. [Nov. i. 



Sandstone predominates in the Pottsville but in later formations 

 shale is a characteristic feature. This is true of the anthracite area 

 almost equally with the bituminous region, as appears from the 

 sections and drill-hole records published by the Pennsylvania 

 survey.^® 



The anthracite region, including the broad eroded spaces be- 

 tween the coal fields, is about 70 by 100 miles, but the coal area, 

 which has escaped erosion, is considerably less than 500 square miles. 

 The Pottsville deposits are almost wholly sandstones or conglomer- 

 ates except locally in the Eastern Middle, where the Beaver shows 

 here and there thick beds of shale, 22 to 42 feet thick. Conditions 

 changed gradually after the beginning of the Athens, so that above 

 the Mammoth coal bed, identifiable with comparative certainty 

 throughout much of the area, one finds abundance of shale. At the 

 northeast in the Southern field, one section has 194 feet of shale 

 in a total of 218; another has one bed, 64 feet, and three thinner 

 beds in 257 feet; midway in the field, some sections show nearly one 

 half shale in 900 feet above the IMammoth. There is much varia- 

 tion in thickness and in position of the shale beds and none is 

 persistent in all the sections. In the Western Middle, the thickest 

 bed at the easterly end is but 40 feet, but farther west are beds of 

 45 to 107 feet with others of less thickness, while at the western 

 end shale and sandstone are often in equal thickness. The propor- 

 tion of fine shale above the Mammoth is as large as in most of 

 the bituminous region and some of the beds are thicker than any 

 there outside of the central space in West Virginia. Farther south 

 along the eastern border in Alabama, where only the Pottsville 

 remains and the conglomerates are thick and coarse, one finds great 

 beds of finer materials, though argillaceous shale seems to be com- 

 paratively unimportant. 



Sandy shales are closely related in distribution to the sand- 

 stones; black or carbonaceous shales will be considerd in connection 



°'' The survey of the anthracite fields was planned by C. A. Ashburner 

 and was executed under his direction. His death occurred after comple- 

 tion of the work but before preparation of the report. Discussion of the 

 result's was assigned to A. D. W. Smith and it is given in Vol. HI. of the 

 Pinal Report. 



