540 GEOLOGY. 



wise argues for wide-spread emergence before the epoch of the Mill- 

 stone grit, for on a land surface streams may shift materials great 

 distances. 



From its coarser conglomerate phases in the east, the Pottsville 

 conglomerate grades westward into sandstone (Mansfield sandstone of 

 Indiana/ Parma sandstone of Michigan, 2 Gray don sandstone of Mis- 

 souri, 3 and Millstone grit 4 of Arkansas), and with local conglomeratic 

 phases the sandstone persists over the interior. At various points in 

 the east the formation contains thin beds of coal (Fig. 242), showing 

 the local beginnings of the conditions which existed later over wide 

 areas, 5 and in the southern Appalachians it is a chief source of coal. 6 

 The beds of the interior referred to this epoch are thought to repre- 

 sent the later part of the epoch only, while the series in the Appala- 

 chian mountains includes beds referable to earlier stages of the epoch 

 as well. In other words, the area of deposition was extended westward 

 as the epoch advanced. Even at the east there is sometimes a great un- 

 conformity between the Mississippian and the Pottsville series (see p. 508). 



The formation varies in thickness from 1200 feet in the South 

 Anthracite basin of Pennsylvania, to as little as 65 feet in some parts 

 of western Pennsylvania. The unequal thicknesses are due partly 

 to the unevenness of the eroded surface on which it rests, partly to 

 unequal rates of sedimentation, partly to unequal duration of the 

 time of sedimentation in different regions, and partly perhaps to inclined 

 deposition, thinning off shore. Half of the formation in eastern 

 Pennsylvania is thought to have been deposited before deposition 

 began in the western part of the state. 7 The formation has a thick- 

 ness of about 500 feet in Maryland 8 and of 200 to 1500 feet in West 

 Virginia. 9 Further details as to thickness are given on pp. 557-663. 



1 22d, 26th, and 27th Ann. Repts., Dept. of Geol., etc. 

 3 Lane, Geol. Surv. of Mich., especially Vol. VII. 



3 Geol. Surv. of Missouri, Vol. XII, and 22d Ann. Rept., U. S. Geol. Surv., Pt. 2, 

 p. 88. 



4 Geol. Surv. of Arkansas, Vol. V, 1892. 



5 White (I. C.), West Virginia Geol. Surv., Vol. II. 



6 Hayes, 22d Ann. Rept., U. S. Geol. Surv., Pt. Ill, p. 235. 



7 White (D.), 20th Ann. Rept., U. S. Geol. Surv., Pt. II, pp. 755-918, and Browns- 

 ville-Connellsville folio, U. S. Geol. Surv., p. 8. 



8 Prosser, Jour. Geol., Vol. IX, p. 422. Also Martin, Maryland Geol. Surv., Report 

 on Garrett Co., pp. 100-110. 



9 White, West Virginia Geol. Surv., Vol. I, p. 211. 



