ESTIMATES AND CAUSES OF CRUSTAL SHORTENING 33 



produced, and thus there are large extensions of the areas 

 affected. 



This is finely illustrated by many districts of the Piedmont 

 plateau crystalline and semicrystalline rocks. A convenient 

 district in which to see the phenomena is that of New York. In 

 the Manhattan gneiss in the vicinity of New Rochelle, the 

 injected material in many places surpasses in quantity the 

 amount of the original gneiss. Parallel injection is also finely 

 illustrated by many of the districts of pre-Cambrian gneisses in 

 eastern Canada, and western North America. In the latter region 

 one of the most beautiful illustrations is that of the Madison 

 Canyon gneiss in Montana. In these regions the intrusions seem 

 to have occurred when the rocks were rather deep-seated, and 

 doubtless in this zone intrusions are of much greater importance 

 than nearer the surface. However, within a few thousand feet 

 from the surface, extensive intrusions may take advantage of 

 joints, faults, or radiating fractures. This is illustrated by the 

 numerous granite dikes along joints, in the Sierra Nevada granite ; 

 by the dikes of Crazy Mountain, Montana; 1 by the trap dikes of 

 the Triassic of the Connecticut valley ; by the dikes of Cape 

 Ann, Massachusetts, where, according to Shaler, 2 the dikes 

 occupy from 5 to 10 per cent, of the surface of the country ; by 

 the dikes of western Scotland ; 3 and by the dikes of many other 

 areas. 



Besides the parallel and radiating intrusions just considered, 

 great irregular intrusions of material have occurred on a vast 

 scale. Irregular intrusions are especially numerous among the 

 older rocks. The injected material may occupy a large part of 

 the surfaces of the districts affected. The irregular injections 

 of igneous material find lateral space largely by mashing or cor- 

 rugating adjacent rocks, and this causes vertical expansion of the 

 crust. Irregular intrusives may be found in the same districts in 



1 Livingston folio, by Joseph P. Iddings and Walter H. Weed : Geol. Atlas 

 of the United States, No. I, 1894. 



2 The geology of Cape Ann, Massachusetts, by N. S. Shaler : 9th Ann. Rept. 

 U. S. Geol. Survey, 1889, pp. 579-602. 



3 Geological map of Scotland, by Sir Archibald Geikie. 



