5G8 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



Clays. — The clays are a dark red or blue, with some variations of yellowish and purple, 

 and are calcareous and arenaceous. The blue clays are not very common, are nearly always 

 higldy arenaceous, and frequently contain vertebrate remains. The red clays are seen at nearly 

 every outcrop and are often more than 100 feet thick, with probably a few thin layers of sand- 

 stone distributed through the strata. 



Thickness and unconformahility. — ^The slight difference in dip and sudden change in litho- 

 logical character of the Triassic beds from the Permian point conclusively to a break in the 

 sedimentation of the two formations. At some locaHties the Triassic beds are overlain by 

 Cretaceous, but generally by Tertiary material. The Cretaceous escarpment or butts resting 

 on the Triassic beds are often 200 feet thick and mostly limestone. The denuding forces that 

 for an immense length of time were cutting these Cretaceous rocks back toward their present 

 limits must have carried away a great deal of the Triassic before it was covered by Tertiary. 

 The strata thus inclosed between two unconformable beds must of necessity vary in thickness, 

 and so we find it varying from a few feet to nearly 400 feet. Even in localities close together 

 the beds vary considerably in thickness. The average, however, will probably } each 200 feet. 



Stratigraphy. — ^The following classification or grouping is not intended as a correlation 

 with any other Triassic beds, but only to apply to the Dockum beds over the area examined. 

 The Dockum may be divided into three beds, though some localities show more, that are more 

 or less well marked. * * * These three main beds are as follows: A lower bed of sandy 

 clay, which is from to 150 feet thick; a central bed or beds of sandstone, conglomerate, and 

 some sandy clay, which is from to 235 feet thick; an upper bed of sandy clay and some sand- 

 stone, wliich is from to 300 feet thick. While these groups represent the different geological 

 horizons over most of the Triassic area, there is nevertheless at some places a thinning out of 

 one, and a thickening of another, which shows that at the same time the conditions of deposition 

 were somewhat different at different localities. The same geological horizon is, therefore, more 

 or less represented in other beds than that which generally represents it. Then, while these 

 three beds do not absolutely represent geological horizons, they do so approximately and are so 

 well marked as to be of much stratigraphical value. 



These strata are fresh-water deposits and their age is determined on vertebrate 

 remains by Cope. 



I-J 17. NORTH CAROLINA. 



Kerr**^ thus described the rocks of the Dan and Deep River belts, the Triassic 

 of North Carolina : 



The rocks are sandstones, clay slates, shales, and conglomerates, generally ferrugiaous 

 and brick-red, but often gray and drab. The shales are occasionally marly, and these and the 

 sandstones are sometimes saliferous. Many of the beds consist of loose and uncompacted 

 materials and are therefore easily abraded. 



The most important and conspicuous member of the series is a large body of black shales, 

 wliich incloses seams of bituminous coal, 2 to 6 feet. This coal lies near the base of the system 

 in both belts and is underlain on Dan River by shales and on Deep River by sandstones and 

 conglomerates, the latter constituting the lowest number of the series and being in places very 

 coarse. And near the eastern margin in Wake County, where the belt reaches its greatest 

 breadth (some 15 miles), the conglomerates are of great tliickness and very coarse, uncom- 

 pacted and rudely stratified, resembling somewhat the half-stratified drift of the mountain 

 slopes, the fragments often little worn and sometimes 10 and 12 inches in diameter, and evi- 

 dently derived from the Huronian rocks of the hUls to the eastward. The conglomerates of the 

 Dan River belt are among the upper members of the series and are mostly fine and graduate 

 into grits and sandstones. 



The black shales near the base of the system contain beds of fire clay and black-band iron 

 ore, interstratified with the coal. They are also highly fossiliferous, especially on Deep River. 

 Silicified trunks of trees are very abundant in the lower sandstones, as may be seen conspicu- 



