EPICONTINENTAL SEA OF JURASSIC AGE 255 
‘The limestones are nearly all fossiliferous, and of the drab 
color peculiar to the Jura in the west. In thickness they vary 
from a few inches to 15 feet. Three or four in the lower 100 
feet and one or two in the upper third of the formation are espe- 
cially prominent.” 
The formation is said to be overlain by the Dakota sand- 
stone. If this so-called Dakota sandstone is at the same hori- 
zon that it is in the Freeze-Out Hills, and it seems from the 
description very probable that it is, then the Jura so-called 
must include the Como beds. The description of the upper 
part of the formation fits the Como, while the lower part with its 
fossiliferous limestones is very characteristic of the Jura both 
north and south of this area. The Como or its stratigraphic 
equivalent is recognized both north and south of this region and 
there appears no good reason for its absence in this area. 
THE MONTANA AREA 
Castle Mountain.A—Vhe Jurassic formation in this area is less 
than one half the average thickness for the interior. Its maxi- 
mum thickness is only ninety feet. The formation consists of a 
basal sandstone overlain by a dense white limestone. The lime- 
stone layer is highly fossiliferous and contains the following 
well-known Jurassic forms: Astarte packard; Trigonia montanaen- 
sis; Pinna kingt; Pholadomya kingt,; Ostrea sp.; Camptonectes 
extenuatus,;, and Gervillia montanaensts. 
The Jura of this locality rests upon upon the Carboniferous 
and the Red Beds are not represented. It is the belief of the 
writers that the beds are wanting altogether in Montana, or at 
least but sparingly represented. 
Little Rocky Mountains.2—Vhe total thickness of the Jura for 
this region is placed at 100 feet. The beds consist of shaly 
gray limestones which change to impure, marly shales and argil- 
laceous limestones. They rest on limestones of Carboniferous 
age and the Red Beds are again absent. 
™WEED and Pirsson, Bull. 139, U. S. Geol. Surv., 1896. 
* WEED and PIRSSON, JouR. GEOL., Vol. IV, 1896. 
