DEVONIAN AGE. 



279 



largely shallow-water or sand-flat formations, as shown by the ripple- 

 marks, shrinkage-cracks, and oblique lamination ; and they therefore in- 

 dicate, by their great thickness, a subsidence during their progress, to a 

 corresponding extent, and, further, that this subsidence or change of 

 level affected most the Appalachian region. The shallow sea extended 

 westward along the southern border of Lake Erie. But it is probable 

 that, over the larger part of the Interior basin, the land lay mostly 

 above the water level. It is difficult, otherwise, to account for the ab- 

 sence of beds between the Black Shale and the Subcarboniferous. 



4. CAT SKILL PERIOD (12). 



I. Rocks : kinds and distribution. 



The rocks of the Catskill period are shales and sandstones of vari- 

 ous colors, in which red predominates. The sandstones are far more 

 extensive than the shales, and pass into conglomerates or coarse grit- 

 rock, and also into a rough mass looking as if made of cemented frag- 



Fig. 557 A. 



Eeen. — Cyclopteris obtusa. 



ments of hard slate. The upper part is generally a conglomerate. 

 There are ripple-marks, oblique lamination, and other evidences of sea- 

 shore action in many of the strata. Some of the layers are partially 

 calcareous. 



The formation, instead of thickening to the westward, in New York, 



