EPICONTINENTAL SEA OF JURASSIC AGE 253 
From specimens collected on the Santa Clara River two miles 
below Gunlock White determined the following species: Penta- 
crinus astericus M.& H.; and 7rigonia sp. Wh.; from near Kanara: 
Pentacrinus astericus M. & H.; Camptonectes stygius White ; Camp- 
tonectes bellistriatus M. & H.; from the northern part of aquarius 
plateau; Camptonectes platessiformis White; Trigonia montanaensts 
Meek and Gervillia sp. White; from Potato Valley, Diamond 
Valley, and near Gunnison: Pentacrinus astericus M. & H. 
From the geographic distribution of the Jura in this region 
it appears that the Jurassic sea did not extend far south of the 
southern boundary of Utah. It may be assumed also that its 
eastern as well as its western shore lines did not extend in this 
region much beyond the state boundaries. From this point the 
eastern shore line extends farther and farther east crossing the 
northwest corner of Colorado thence continuing toward the 
northeast and including the Black Hills area. 
The thinning out of the beds toward the south may be due 
to the presence of a low land area at the south during this epoch. 
A high land area should give a thick shore deposit of a coarse, 
clastic nature. According to the above statements, however, 
the beds consist of calcareous and gypsiferous shales which 
indicate either a somewhat remote shoreline or a low bordering 
land area. 
THE BLACK HILLS AREA’ 
The Jurassic formation forms one of the members in the rim 
of sedimentary rocks which encircles the crystalline area of the 
Black Hills. Here as in the central and southern areas the Jura 
rests upon the Red beds and is overlain by the Lower Cretaceous, 
the Como beds. Its thickness is in the neighborhood of 200 
feet. It exhibits in general about the same lithological characters 
that are noticeable in the formation in the Southern Wyoming 
area. The beds consist of sandstones, arenaceous shales and 
marls, and thin beds of impure fissile limestone. 
Whitfield? has determined the following species from this 
*JENNEY: Nineteenth Ann. Rep. U. S. Geol. Surv., p. 593. 
? Geology of the Black Hills, 884. 
