NON-MARINE PROGRESSIVE OVERLAP 631 



three-fourths of an inch in length or over. In this region* interbedded 

 fossiliferons shales occur, showing the proximity of the sea and its occa- 

 sional invasion of the growing Pottsville fan. In Huntingdon and Bed- 

 ford counties, Pennsylvania, where the thickness of the formation is from 

 1,100 to 1,200 feet, and where it is mostly sandstone, a shaly layer with 

 marine fossils (Spirifer, Ehynchonella, and productoid forms) has been 

 discovered 400 feet above the base of the series. This and the Maryland 

 sections are about in a line parallel to the front of the growing fan, as 

 shown by the correspondence of the thickness. 



In tracing the Pocono northwestward, we find that in the northwestern 

 part of Lycoming county, Pennsylvania, it consists of 665 feet of current- 

 bedded sandstones, with a one-foot seam of coal lying 80 feet above its 

 base. In the northwest of Lycoming county its thickness is reduced to 

 350 feet and it is still a current-bedded sandstone. In Potter county, 

 Pennsylvania, we have 330 feet of sediments, with a thin layer of coarse 

 sand or conglomerate, the Shenango, at the top. Westward from this the 

 pebbles become flat, like those of the underlying Chemung sandstones. 

 In McKean county, Pennsylvania, the Pocono is not much over 200 feet 

 thick, the Shenango being 40 feet. In most of these northwestern sections 

 intercalated strata with marine fossils show the presence of the Waverly 

 sea, which laved the front of the great Pocono fan and into which its 

 edge dipped. Downward these beds grade into fossiliferous Lower Car- 

 bonic strata, which are the contemporaneous deposits of the eastern end of 

 the Waverly sea. The margin of this sea was gradually pushed westward 

 by the growing fan, as shown by the character of the deposits. 



If we glance for a moment at the contemporaneous marine deposits of 

 the Ohio-Michigan area we find that the coarseness of the material 

 decreases toward the northwest. Many sandstone beds, like the Berea, 

 die out in northern Michigan, while beds like the Logan change from 

 conglomerates to sandstones. This indicates the Appalachian source of 

 the material, even of the marine deposits, showing that the streams which 

 built the subaerial fan also supplied the material for the bordering marine 

 strata. 



In the summary given by Stevensonf of the Pocono, the vital fact is 

 brought out that the thinning of the Pocono in northwestern Pennsyl- 

 vania and in West Virginia is due "apparently in part" to "loss of the 

 lower beds." The significance of this fact is best stated in Professor 

 Stevenson's own words : 



"The Pocono of Pennsylvania, Ohio, Kentucky, and Virginia has been re- 

 garded by most geologists as Lower Carboniferous throughout The Pocono of 



• Maryland Geological Survey, Garrett county, p. 168. 

 f Stevenson: Loc. cit., pp. 39, 40. 



LVII— Bull. Gbol. Soc. Am., Vol. 17, 1905 



