SUBCARBONIFEROUS PERIOD. 309 



200 feet thick, of greenish sandstone. (Lesley.) Some thin layers consist of an 

 impure, rough-looking limestone. 



This red-shale formation is 3000 feet thick at the Lehigh, Schuylkill, and Sus- 

 quehanna Rivers j but on crossing the Coal measures to the westward it rapidly 

 diminishes. At Broad Top it is less than 1000 feet; at the Alleghany Mountain, 

 hardly 200; at Blairsville, 30 feet; and beyond, it is lost to view. (Lesley.) 



The soft shales retain still the ripple-marks from the ancient waves, and rain- 

 drop impressions from the showers of the day. The amphibian footprints 

 described beyond are from this formation. 



Seams of coal occur in the Subcarboniferous at many places in Pennsylvania 

 and Virginia. In Montgomery co., Va., there is a layer of coal, two to two and 

 a half feet thick, resting on a bed of conglomerate ; and 30 to 40 feet higher 

 there is another layer, six to nine feet thick, consisting of alternations of coal 

 and slate. These coal beds occur in the Lower group, and are covered by 

 the shales of the Upper. In Pennsylvania, there is a coal bed (and possibly 

 two) in the same Lower group at Tipton, at the head of the Juniata, 600 feet 

 below the Upper shales; but, as far as known, it is a local deposit (Lesley). 

 The Subcarboniferous coal deposits are sometimes called false coal measures. 



(e.) Eastern border region. — In Nova Scotia, the Subcarboniferous rocks are 

 red sandstones and red and green marls, with thick limestones full of fossils. 

 The estimated thickness is 6000 feet. To the north, towards the Azoic, the lime- 

 stones fail, and, instead, the rocks are to a greater extent a coarse conglomerate. 

 To the south, limestones prevail. Beds of gypsum accompany the limestones. 

 The localities of these beds, mentioned by Dawson, are the Carboniferous 

 districts of northern Cumberland, Pictou, Colchester and Hants, Richmond 

 county and southern Inverness, Victoria, and Cape Breton. 



In the lower part of these Subcarboniferous beds, as in those of Virginia, 

 there are, on a small scale, false coal measures, and in one instance a bed of erect, 

 trees, under-clays, and thin coal s-eams; and the same beds contain numerous 

 remains of fish. 



The fish-bearing shales of Albert Mine, New Brunswick, are referred to this 

 period by Dawson, from whom the above facts are cited. This mine affords a 

 peculiar pitch-like or asphaltic coal, which has been regarded by some investi- 

 gators as a true coal seam, much altered in the course of the violent folding and 

 metamorphic changes to which the rocks of the region, as all admit, have been 

 subjected ; and by others as an inspissated mineral oil filling a fissure in the beds. 

 The coal is mentioned on p. 68, and analyses are given on the following page. 



In Pennsylvania, two epochs may be distinguished in the Subcarboniferous, — 

 namely, the Alleghany, corresponding to the lower beds, and the Schuylkill, cor- 

 responding to the upper beds. It is probable that the two divisions in Tennessee 

 (p. 308) are equivalents of these, and that the Alleghany epoch corresponds to the 

 lower of the Western Subcarboniferous limestones, the Burlington and Keokuk, 

 with the underlying beds. 



