Marsh — Jurassic formaUon on the Atlantic Coast. 441 



Position of Jurassic Strata. 



In the geological section, figure 1, on page 434, the relative 

 position of the Jurassic deposits of the West is designated, 

 and this will hold good for all the strata of that age in known 

 localities on both flanks of the Rocky Mountains. In the 

 East, the position of the deposits here regarded as Jurassic 

 is equally definite, and corresponds strictly to that of the 

 western horizon in its most essential features. A reference 

 to the section in figure 2, below, will make this clear. This 

 typical section is based on one by G. H. Cook, in the Geology 

 of New Jersey,"^ and represents the successive Mesozoic and 

 more recent formations, from New Brunswick, New Jersey, 

 on a line southeast, through Lower Squankum to the Atlantic. 

 The relative proportions and inclination of the various divisions 

 cannot, of course, be given accurately in so small a figure. The 

 distance represented by this section is about forty miles. 



Figure 2. — Geological Section in New Jersey, 

 a, Triassic ; &, Jurassic ; c, Cretaceous ; c?, Tertiary ; T, tide level. 



In this section, the red Triassic shales and sandstones are shown 

 on the left, highly inclined. Resting on their eroded surface 

 are the well-known variegated plastic clays, also of fresh-water 

 origin. These strata are nearly horizontal, having a slight 

 inclination toward the ocean. The top of these peculiar clay 

 beds is not clearly defined, but is marked by a change from 

 lacustrine to marine conditions, which clearly indicate deposi- 

 tion in water of increasing depth, and finally deep-sea glauco- 

 nite strata. These greensand deposits continued with some 

 interruptions throughout the remaining Cretaceous time, and 

 even into the early Tertiary, the third, or upper, marl bed 

 being Eocene. Over these are strata of Miocene age, the 

 Ammodon beds, and still more modern deposits form the shore 

 of the Atlantic. 



Atlantic Barriers. 



The change from the fresh-water plastic clays of New Jersey 

 to the marine beds containing greensand over them proves not 

 only the breaking down of the eastern barrier which protected 

 the former strata from the Atlantic, but a great subsidence 

 also, since glauconite, as a rule, is only deposited in the deep, 

 still waters of the ocean. 



* Geological Map, Cretaceous Section 3, 1868. 



