4 86 



THE TRIASSIC PERIOD 



Kinds of rock. The rocks of this series l include all the common 

 varieties of fragmental rocks, some of which are developed in un- 

 usual phases. Sandstones and shales predominate, but conglomer- 

 ates and breccias are present, and, locally, limestone and coal. 



Conglomerate lies at the base of the system in many places, and 

 is made up chiefly of material from the underlying crystalline 

 schist. Conglomerates also are the border phase of beds which 

 grade laterally into sandstone, and even into shale. The chief 

 constituent of the conglomerate is quartz, the most resistant part 

 of the underlying terranes; but locally, quartzite, crystalline schist 

 and limestone are severally its principal constituents. 



Sandstone and shale make up the great body of the Newark 

 series, and both possess distinctive characteristics. The prevalent 

 color is red, though there are shales which are black, and sandstones 

 which are gray. Some of the sandstone is arkose, that is, contains 

 feldspar, and both sandstone and shale contain much mica. Both 

 these constituents abound in the metamorphic rocks from which 

 the Newark sediments were chiefly derived. Except locally, the 

 series is poor in fossils. 



Conditions of origin. The character of the Newark formations 

 and their fossils, mainly land plants, footprints of reptiles, and 

 fresh- or brackish-water fishes, indicate that they are of continental 

 rather than marine origin, but do not tell the precise manner in 



Fig. 413. Diagram showing the development of a trough, now partly filled by 

 sediment, by warping. 



which they were laid down. The depressions in which these beds 

 were deposited may have been due to warping or to faulting, or 

 partly to the one and partly to the other (Figs. 413 and 414). How- 

 ever formed, they became the sites of lakes, bays, estuaries, dry- 

 basins, or aggrading rivers. 



The considerable thickness of the sediments, taken in connection 



1 The Connecticut valley and New York- Virginia areas are best known, and 

 the descriptions of the formations here given apply especially to them. 



