812 



SCIENCE. 



[N. S. Vol,. IV. No. 101. 



greensand deposits continued with some in- 

 terruptions throughout the remaining Cre- 

 taceous time, and even into the early Ter- 

 tiary, 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 Atlan- 

 tic. 



ATLANTIC BARRIERS. 



The change from the fresh- water plastic 

 clays of New Jersey to the marine beds con- 

 taining greensand over them proves not 

 only the breaking down of the eastern bar- 

 rier 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. 



The Miocene greensand surmounting the 

 Gay Head clay cliffs on Martha's Vine- 

 yard also means the same thing, and a still 

 greater lapse of time, as the whole Creta- 

 ceous and Eocene strata are here apparently 

 absent. The present height of these Mio- 

 cene strata indicates indirectly the mini- 

 mum 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 east- 

 ern 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 ISTew 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 



*Geology of New Jersey, Keport on Clays, p. 30, 



1878. 



rightly, I saw there the typical clays^ 

 lignites, and iron ores, that mark the hori- 

 zon now under consideration. South of 

 New England, the inner barrier is equally 

 well defined by the Triassic and older rocka 

 to the Potomac Eiver, 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 con- 

 ditions under which they were deposited. 

 The close correspondence in this respect be- 

 tween 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 

 black 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 Bocky 

 Mountain region the brilliant hues of the 

 Jurassic strata may be seen for miles on the 

 face of the high bluffs. This is especially 

 remarkable in the cliffs at Como, Wyoming, 

 a representation of which is before you. 

 Still more brilliant effects may be seen in 

 the canyons on the west side of the Green 

 Biver, in eastern Utah. 



East of the Bocky 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 similar exhibition is to be seen, 

 as represented in the second drawing. Thia 

 is repeated on a much larger scale further 

 south, near Caiion City, Colorado, as like- 

 wise shown in still another sketch, but 



