320 The American Geologist. Novcmbor, isaa 
tion to the vertical depth of nearly a iiiiie reduced all the surrouadins; 
couutry to a base-level. 
The head stream of Shields river, a tributary of the Yellowstone, has 
cut a deep vallej^ in the western side of the range, dividing it into 
southern and northern halves. The southern part, which is the higher, 
has numerous sharp, jagged peaks, from which the streams flow radially- 
outward to the west, south, and east. In advancing up any of the val- 
leys, it is found at first comparatively broad and bounded by high blufts 
of nearly horizontal sandstones; on approaching the central peaks, the 
valley becomes narrow, and the stream descends from a higher level 400 
or 500 feet by cascades and falls: and beyond this the valley again 
widens somewhat with a more gentle slope to its head. The "fall-line" 
is found on all the radial streams, and is due to the local hardening of 
the sedimentarj^ Laramie rocks by a central stock of coarsely crystalline 
diorite, which is irregularly oval in outline and about six miles wide at 
its greatest diameter. From their contact with this stock the Laramie 
strata, dipping gently outward, are hardened and metamorphosed to a 
distance of about a mile. The central portion of the diorite is cut by 
masses of a light-colored, finer-grained granitite. 
"It is surprising," writes Dr. Wolff, "to see the similarity between this 
Tertiary diorite and granite and the Paleozoic masses of similar rock 
found exposed on the old eroded surfaces of the Atlantic states, as, for 
instance, on the northern shore of Boston. In both cases the same 
black patches are seen in the granite, referable here to enclosed dioritic 
fragments, and the same alternations of basic and acid rock in streaks 
or 'Schlieren' with parallel flow structure. * * * * The diorite 
.stock as well as the adjacent Cretaceous rocks are cut b}- later vertical 
dikes of diorite-porphyrite and allied rocks; these dikes swarm in the 
contact zone, accompanied by horizontal and oblique sheets of similar 
rock. Mr. J. P. Iddings, who visited this place in 1891, finds that the 
vertical dikes, both in the stock and in all this part of the range, have a 
general radial arrangement, with the diorite mass as an approximate 
center, repeating a fact observed by him in a smaller diorite stock in 
the Yellowstone mountains. These long radial dikes extend outward 
even into the benches at the southern base of the range." 
In the northern half of the range and its outlying buttes, the most 
prominent of tlie eruptive rocks, first collected by Dr. \Yolff in 1883, is 
"found to be composed of feldspar (iu part triclinic), augite andnephe- 
line, with biotite, sodalite, magnetite, olivine, a>girine, etc., accessory; 
as an abyssal intrusive rock with the mineral combination nepheline, 
soda-lime feldspar, it filled a gap in the classification of Professor Rosen- 
busch, and was called by him 'theralite', as the first undoubted represen- 
tative of this family." 
In conclusion, the author speaks of these mountains as an easily ac- 
cessible and magnificent field for further geologic and petrographic in- 
vestigation. "From Livingston or adjoining stations on the Korthern 
Pacific railroad it is an easj' day's drive to the foot of the range; the 
canyons of the larger streams on the east side are easily accessible by 
