TRIASSIC. 527 



The Stockton formation comprises gray to buff arkosic sandstone, red-brown sandstone, 

 and numerous masses of red shale in no regular succession and presenting many local variations 

 in stratigraphy. It lies on gneiss, schist, quartzite, and limestone, in Montgomery, Bucks, 

 and northern Chester counties. Many of the sandstones are cross-bedded, and the finer-grained 

 rocks exhibit ripple marks, mud cracks, and raindrop impressions, which indicate shallow- water 

 conditions during deposition. The lower beds usually are arkose — a sandstone containing 

 more or less feldspar and kaolin derived from the gneiss. The Ijockatong formation consists 

 mostly of dark-colored, hard, compact, fine-grained rocks originally composed of a mixture of 

 clay and fine sand in variable proportions. Some beds are moderately massive, others vary 

 from shaly to flaggy, and many of them exhibit mud cracks and other evidences of shallow-water 

 deposition. Some of the beds contain carbonaceous material. The Brunswick shale consists 

 mainly of a great thickness of soft red shale with some thin sandstone layers. 



These three formations are not sharply separated by abrupt changes of materials but usually 

 merge through beds of passage which appear to vary somewhat in thickness and possibly also 

 in stratigraphic position in different areas. 



Clark "^ gives the following brief account of the Newark formation in Maryland. 

 The belt passes thence into Virginia. (For a discussion of the Richmond field, 

 see pp. 518-519.) 



The deposits of the Newark formation unconformably overlie the limestone and phyllite 

 which have been above described and cover a considerable area along the western border of the 

 Piedmont Plateau. Beginning as a belt some 10 miles in width in northern Carroll and Fred- 

 erick counties, the formation gradually narrows toward the south, until in the region of Frederick 

 its fuU width does not exceed 1 mile, while at one point directly to the west of Frederick the 

 continuity of the beds is completely broken. Farther southward, in western Montgomery 

 County, the belt of Newark deposits again broadens to a width of several miles. 



The rocks of the Newark formation consist largely of red and gray sandstones and conglom- 

 erates of both siliceous and calcareous varieties. The finer-grained and deeper-colored deposits 

 generally have their individual elements united by a ferruginous cement, while the calcareous 

 conglomerate, which is largely made up of rounded limestone pebbles, is generally embedded 

 in a reddish calcareous matrix. All of the deposits present structures which indicate that they 

 were formed in shallow water; the coarse conglomerates, the ripple-marked surfaces, and the 

 tracks of animals all point indisputably to this conclusion. 



J-L 10-12. WESTERN UNITED STATES. 



A philosophic discussion and correlation of American Triassic stratigraphy- 

 has been given by J. P. Smith."^ The marine Triassic of the western United States 

 is found in Oregon, Idaho, California, and Nevada. Its distribution and relations 

 are summed up by Hyatt and Smith *'*^ as follows: 



The marine Triassic section of [western] America is unusually complete, and its thickness 

 compares favorably with that of any other region. All three subdivisions — Lower, Middle, 

 and Upper Triassic — are represented by calcareous deposits, aggregating approximately 4,000 

 feet in thickness. Of this amoimt, about 800 feet belong to the Lower Triassic, about 1,000 

 feet to the Middle Triassic, and about 2,000 feet to the Upper Triassic. 



The entire section is not represented at any one locality, nor is the thickness of each division 

 constant. Furthermore, the marine Triassic is not everywhere developed as a calcareous 

 formation. In the United States the Triassic system is represented by marine deposits only 

 in the Western States, in Idaho, Nevada, Oregon, and California. 



The Lower Triassic is known only in Idaho and southeastern California, where about 800 

 feet of shales and limestones contain fossils characteristic of this series. The most important 

 genera are Meekoceras, Danubites, Columbites, Flemingites, Aspidites, Lecanites, Ophiceras, 



