442 MdTsh — Jurassic formation on the Atlantic Coast. 



The Miocene greensand surmounting the Gray Head clay 

 cliffs on Martha's Yineyard also means the same thing, and a 

 still greater lapse of time, as the whole Cretaceous and Eocene 

 strata are here apparently absent. The present height of these 

 Miocene strata indicates indirectly the minimum of elevation^ 

 the depth of the sea in which they were deposited being at 

 present one of the unknown elements. It has been suggested 

 by some geologists that the eastern barrier was composed of 

 granitic rocks, and thus furnished the materials for the New 

 Jersey and other clays.* Many known facts support this view. 



The western, or inner, barrier of this great fresh-water border 

 lake is still well marked. In the New England region, the 

 present rock-bound coast line indicates its approximate position, 

 and retains in its bays and inlets remnants of the deposits then 

 laid down. Away from the coast, I know of only a single 

 locality that seems to have preserved these beds, and that is 

 near Brandon, Vermont. This basin I explored long ago, and 

 if my memory serves me rightly, I saw there the typical clays, 

 lignites, and iron ores, that mark the horizon now under con- 

 sideration. South of New England, the inner barrier is 

 equally well defined by the Triassic and older rocks to the 

 Potomac River, but beyond that point I have not carefully 

 examined it. 



Physical Characters of the Jurassic. 



The strong resemblance in their physical characters between 

 the fresh- water deposits here regarded as Jurassic and those long 

 known to be such in the Rocky Mountain region is largely 

 dependent on the materials of which they are formed, and the 

 conditions under which they were deposited. The close corre- 

 spondence in this respect between the beds of the two regions 

 should have some value in estimating their age. 



The most striking feature in these deposits is the variety of 

 colors in the plastic clays. Brilliant red, green, and yellow 

 tints are especially prominent, yet the white and blacli shades 

 are equally noticeable. While these colors are often seen in 

 great masses, marking definite strata in fresh exposures, they 

 blend one with another from the effects of weathering, where 

 the original colors wash over each other. In the Rocky 

 Mountain region, the brilliant hues of the Jurassic strata may 

 be seen for miles on the face of the high bluffs. This is espe- 

 cially remarkable in the cliffs at Como, Wyoming, a represen- 

 tation of which is before you. Still more brilliant effects may 

 be seen in the canyons on the west side of the Green River, in 

 Eastern Utah. 



East of the Rocky Mountains, the same color scheme is well 

 illustrated around the Black Hills, in South Dakota. Again 

 in the foot-hills west of Denver, near Morrison, Colorado, a 



* Geology of New Jersey, Report on Clays, p. 30, 1878. 



