ON THE NATURE OF IGNEOUS INTRUSIONS 187 
as have others, of the nature of anticlinals produced by lateral 
compression. He says:* . 
In general terms, while the details are extremely complicated, we may 
express the structure of a belt of country known as the Sawatch range (in 
east-central Colorado), eighty miles in length from north to south, and at least 
forty from east to west as a single wedge of granite, thrust upward, and the 
sedimentary rocks inclined from either side. The illustration of which the 
Sawatch range is the central mass is probably on a grander scale than any 
other in the West, but there are abundant examples of smaller size. The 
Black Hills of Dakota, the Laramie range, Big Horn, Wind River and many 
others are of the same type. 
The cores exposed in the Black Hills, Big Horn and other 
mountains referred to above, are of granite. Whether these 
granitic centers answer to the plutonic plugs in the smaller domes 
to the northward of the Black Hills, or belong structurally to 
the sedimentary beds upturned on their flanks, and have them- 
selves been elevated by intrusions beneath, is not clear. I am 
inclined to the hypothesis, however, that the granite was the floor 
on which the sedimentary beds were deposited and that it has 
itself been elevated with them. 
The magnitude of the elongated domes from which the 
present mountains of Colorado and Wyoming have been sculp- 
tured, might perhaps be urged as an objection to the view of 
their origin here expressed. As it will tend to make us more 
cautious if the size of the problem we have attacked 1s fully 
realized, let us endeavor to form a mental conception of the 
region referred to as it would appear under this hypothesis, if 
unaffected by erosion. 
On the summit of Mt. Lincoln, Colorado, 14,297 feet above 
the sea, there are, according to Hayden, clastic beds belong- 
ing to the same series as the strata upturned on the flanks of 
the mountains of which Mt. Lincoln is one of the dominant 
peaks. No one who is familiar with the geology of Colorado 
will dispute, I fancy, that this and other similar evidence, may 
be taken as proof that the granite now forming the central 
tU. S. Geological and Geographical Survey of the Territories, annual report for 
the year 1873, p. 49. 
