GEOLOGY OF LITTLE ROCKY MOUNTAINS 409 
the base of a series of leaden blue shales, which vary in their 
’ 
shale character- 
istic of the Pierre beds, so common in the plains country south 
character and do not appear to be the ‘gumbo’ 
of the mountain. These shales carry a few fossils and are over- 
lain by other shales carrying sandstone concretions, from which 
numerous fossils were obtained and which were kindly identified 
by Mr. Stanton. 
Gontobasis sublevis M. and H. 
Corbicula cytheriformis M. and H. 
Ostrea sp. . 
This shale series is in turn capped by white, porcellanous 
beds, in which there are abundant impressions of fish scales. 
This rock weathers into a sherdy, porcelain-like débris, whose 
light color attracts attention whenever the rocks are exposed. 
IGNEOUS ROCKS. 
Distribution.—lgneous rocks cover a considerable part of the 
Little Rocky Mountain region, forming the central area and 
occurring in several marginal buttes, the distribution being shown 
on the accompanying geological map (Fig. 2, p. 403). The rocks 
are porphyries, belonging to the granite-syenite series, and are of 
highly alkaline types passing into the phonolites, one rock 
belonging to the latter family occurring near Landusky. 
Occurrence.—The porphyry occurring in the central area of 
the mountains belongs to a single large body, intruded con- 
formably in the arch of the uplift between the crystalline schists 
and the Cambrian limestones, the soft shales of the latter forma- 
tion affording an easy horizon of fracture and intrusion. This is 
shown in the diagrammatic cross section of the mountains (Fig. 
2, p. 403) in which the porphyry is represented as a single intru- 
sive mass, thickened on the summit of the low flat arch and 
thinner at the edges. This seems to be the actual occurrence 
in the mountains, though the precise thickness between the 
limestones and crystalline schists was not measured. This makes 
the intrusion laccolithic in character. 
The cross section shows a thinning of the laccolith to the 
