ZONES OF INTRUSION 409 



associated rocks were formerly buried to a depth at which intrusive 

 masses are more generally present than they are at the surface, and in 

 relation to our subject, it is merely one of the evidences of decided eleva- 

 tion and pronounced erosion of the positive elements. 



There are, however, zones in which igneous intrusions are peculiarly 

 voluminous in rocks that never were so deeply buried. Some of these 

 occur between a positive and a negative element or between two negative 

 elements; others, as the Cordillera, are widely distributed without appar- 

 ent order. 



The Kew England-New Brunswick-lSTewfoundland zone, which in fact 

 extends from Philadelphia to Saint Johns, ISTewfoimdland, is one of the 

 first kind. It lies between Laurentia, to which we must reckon the Adi- 

 rondacks and Highlands, and the Atlantic, i. e., between two of the largest 

 elements we can distinguish, one of which is very decidedly positive and 

 the other pronouncedly negative. The d3'namic activities in this zone 

 have repeatedly been marked by igneous intrusion and extrusion from 

 early Proterozoic time to the Triassic. In explanation of this condition 

 we may point out that the zone between the sinking Atlantic element and 

 rising Laurentia was a zone of displacements, in which temperature and 

 pressure must vary notably, and consequently conditions would arise 

 favorable to the formation of molten masses.* 



These conditions distinguish the New England province from the 

 Southern Appalachian, where the absence of vulcanism, except in the 

 late Proterozoic and Triassic epochs, is a conspicuous and unexplained 

 fact. 



In the Lake Superior region volcanic and plutonic activity was fre- 

 quent on a very large scale until the close of the Keweenawan, when it 

 ceased apparently forever. During the periods of activity the region had 

 a negative tendency and lay adjacent to the Laurentian and Wisconsin 

 elements. When the vulcanism was finally extingiiished the negative 

 element had become welded to the positive in consequence of tangential 

 thrust, and it has since moved with them. 



Throughout the great western Cordillera intrusive and extrusive rocks 

 are so widely distributed that it is apparent no one simple condition of 

 continental structure controlled their appearance. The widespread and 

 obvious igneous masses have, however, been erupted since the Triassic — 

 that is, since the continent acquired a certain unity and since the develop- 

 ment of those hypothetical deep-seated shear zones which seem essential 

 to the eastward movement indicated by folding. With these relations 

 in mind, we may suggest that the Mesozoic and younger intrusions bear 



• p. G. Talt : Heat, 1884, p. 123. 



