ESTIMATES AND CAUSES OF CRUSTAL SHORTENING 49 



present purposes it is not necessary to enter into this discussion, 

 but I wish to recall the facts as to the dominance of intrusives. 

 Intrusive rocks are discoverable only after a region has been 

 eroded, and it is therefore in these denuded regions that we are 

 to look for evidence of intrusion. Beginning with the older 

 periods, and confining our attention to America, we find that the 

 Archean, so far as we can ascertain its original character, consists 

 largely of modified plutonic rocks. Passing to the Algonkian, 

 hardly an area is found in which intrusive rocks do not occupy 

 a large percentage of the area. This is illustrated by the great 

 masses of intrusive rocks in the Lake Superior region, which in 

 many districts occupy large fractions of the areas. In the Rocky 

 mountain region, in various districts, the Algonkian sedimen- 

 tary rocks are subordinate to the simply enormous quantities of 

 intrusive granite and other rocks. This is well illustrated by the 

 Pikes Peak district, where, according to Cross, 1 the intrusive 

 granite occupies two-thirds or three-fourths of the entire area 

 of the one-half-square-degree quadrangle, and where the Algon- 

 kian sediments are mere fragments ; by the Black Hills ; by the 

 Medicine Bow mountains, and many other ranges. Passing to 

 the Paleozoic and Mesozoic, in almost every mountain region 

 there are enormous masses of intrusives. This may be illus- 

 trated by the great batholiths 2 of granite in the Sierra Nevada 

 and in the New England regions, by the laccoliths of the Henry 

 mountains, of the Elk mountains and La Plata mountains, and 

 by irregular intrusions, sills, and dikes, in almost every mountain 

 district in the country. As yet the known Tertiary intrusives in 

 America are not so important, but in Great Britain, where denu- 

 dation has gone far, a vast quantity of the Tertiary intrusives has 

 appeared. Doubtless in America also, when denudation shall 

 have advanced far enough, correlative with the volcanics men- 

 tioned p. 48 will be found a great quantity of intrusive rocks. 



1 Pikes Peak folio, by Whitman Cross : Geol. Atlas of the United States, No. 7, 

 1894. 



2 Suess's term batholith is here used in its strict etymological sense, with no 

 reference to any theory as to how the magma was transferred, or as to whether or not 

 it occupied previously existing spaces. 



