254 W: N. LOGAN 
area: Asterias dubium Whitt.; Pentacrinus astericus M. & H.; Lin- 
gula brevirostris M. & H.; Rhynchonella myrina M. & H.; Ostrea 
strigilecula ‘White; Gryphea calceola, var. nebrascensis M. & H.; 
Pecten newberryt Whitf.; Camptonectes bellistriatus M.; Campto- 
nectes extenuatus M. & H.; Pseudomonotis curta Hall; Pseudomono- 
tis orbiculata Whitf.; Avicula (Oxytoma) mucronata M. & H.; 
Gervilta recta M.; Grammatodon inornatus M. & H.; Mytilus 
whiter Whitf.; Volsella (Modiola) formosa M. & H.; Volsella per- 
temus M. & H.; Astarte fragilis M. & H.; Trapezium belle- 
fourchensts Whitf.;  Tvrapezium subequalis Whitf.; Pleuromya 
newton. Whitf.; Zancredia inornata M.& H.; Tancredtia corbulh- 
formis Whitf.; Tancredia bulbosa Whitt.; Tancredia postica Whitt ; 
LTancredia warrenana M. & H.; Dostna gurassica Whitt.; Psammo- 
bia? prematura Whiti.; Thracia? sublevis M. & H.; Neaera 
longirostra Whitf.; Saxtcava jurassica Whitf.; Quenstedtoceras 
( Cardioceras) cordiforme M. & H.; and Belemnites densus M. & H. 
In the Big Horn Basin region Eldridge* discusses the Jura 
as follows: ‘This, so far as the evidence obtained indicates, is, 
within the region under examination, wholly of marine origin. 
The thickness is between 400 and 600 feet, which is approximately 
maintained over the entire area of exposure. Shales constitute 
the mass of the formation in which from base to summit occur 
thin beds of sandstone and fossiliferous limestone of types char- 
acteristic of the Jura in the Rocky Mountain region. Gray is the 
predominating color of the shales, but throughout the formation 
red, purple, yellow, slate, and pink, in greater or less intensity, 
may be observed. At a number of localities a considerable 
amount of siliceous matter appears, in occurrence suggesting the 
action of hot waters. 
‘The sandstones are of slight importance. They are chiefly 
gray with a slight greenish tint. The lower beds, however, are 
red, shaly and transitional from the Trias, while near the sum- 
mit are two of greater thickness, which, but for their tint and 
the overlying typical Jura shales, might be confounded with the 
Dakota. 
tBull. U. S. Geol. Surv. No. 119. 
