32 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 69 



extremely low relief of the region in Jurassic time renders this 

 explanation improbable. 



(3) Water of the Jurassic sea may have found its way into the 

 lowest places on the partly submerged peneplain by more .or less 

 circuitous routes, and because of poor connection or perhaps because 

 of intermittent connection with the sea, evaporated and deposited 

 gypsum. Some of these partly inclosed arms of the sea must have 

 received enough fresh water from the drainage of the surrounding 

 country to prevent precipitation of gypsum, or even to keep the 

 water essentially fresh, hence the occurrence of fresh-water lime- 

 stone and non-gypsiferous clastic sediments in some places at a 

 horizon which seems to be the same as that of the gypsum beds. 

 The absence of salt from the gypsiferous middle La Plata indicates 

 that concentration in the arms of the sea did not reach so advanced a 

 stage as it did in the older seas in this region. In this connection 

 it may be pointed out that some of these arms should have contained 

 water suitable for the marine life that flourished in the main body 

 of the sea. Doubtless the boundary of this sea, shown in figure I, 

 p. 11, will be extended as new information is obtained, and it is 

 possible that careful examination will disclose the presence of marine 

 fossils where they have not yet been found. 



(4) The gypsum deposits, although at nearly the same horizon, 

 may differ slightly in age, and the several deposits represent tempo- 

 rary basins partly or wholly cut off from the broad but very shallow 

 sea. Such basins would be formed readily on the partly submerged 

 peneplain by slight warping of the surface; by sand bars; by vege- 

 table growth ; or in other ways. 



Little need be said in this connection of the occurrence of lime- 

 stone where marine fossils are found nor of the ordinary fresh- 

 water limestone which occurs in thin beds in the La Plata group and 

 in the McElmo formation. But there are some limestones which are 

 quite different from the others, in that they are dark-colored and 

 bituminous, and which by reason of their peculiar nature are easily 

 recognized. They seem to be confined to a very narrow zone and 

 hence are valuable horizon markers. The dark-colored limestone and 

 limy shale occur in the middle of the La Plata group- in southwest 

 Colorado and are described as being easily recognized by their litho- 

 logic character. At the same horizon in southern Colorado, in 

 Piedra Valley, this limestone is dark-colored and bituminous, and is 

 shaly in some places and brecciated in others. Farther south in New 

 Mexico a thin, dark-colored, bituminous, shaly limestone underlies 



