MANNER OF INTRUSION. 709 



masses are concerned, is as if a solid body were pushed up against a sheet 

 of rubber. The rubber would be thinned and extended above the mass 

 and on the sides. Therefore a consequence of recrystallization is to carry 

 the entering magma nearer to the surface. 



The deep-seated intrusives, producing uplift of the intruded rocks, form 

 mountains at the surface. Such mountains have been called subtuberant 

 by Russell and others." When later erosion removes the capping rocks the 

 igneous masses below are seen. Intrusive igneous masses of this class are 

 exposed on a great scale in this country, in the Cordilleras, in the Black 

 Hills, northwest of Lake Superior in Canada, and in New England. The 

 intrusive masses may be nearly spherical, lenticular, or many times longer 

 than broad. The larger of them have minor diameters of many kilometers 

 and major diameters of 50 to 100 kilometers, or even more. When the 

 magnitude of these masses is appreciated it is easy to understand how the 

 great interior earth stresses result in concentrated upward motion at some 

 area, as a Consequence of which the magma, either . liquid or potentially 

 liquid, slowly but with tremendous power oozes up toward the surface, 

 carrying with it and pushing aside the superincumbent rocks. 



(2) Next in importance to raising the intruded rocks and pushing them 

 aside in the lower zone is the breaking across or the following of the 

 structures or planes of weakness of the intruded mass. During the great 

 movements resulting in mass intrusion the rocks which are thrust aside are 

 cut and injected parallel to their structures on a large scale, as a necessary 

 correlative of the greater process above described. Returning to the 

 illustration of the leather, this material ' stretched against a hemisphere is 

 wrinkled on its sides unless it be as extensible as rubber. Similarly, as a 

 great mass of igneous rock is intruded and pushes the rocks up and aside, 

 they are wrinkled, unless we assume greater flexibility than is warranted 

 by the facts. Commonly, therefore, subtuberant intrusion demands close 

 corrugation on the sides. Further, I have held in another place that great 

 periods of intrusion are also periods of rapid orogenic movements.'' 



When the deformation of a rock mass is rapid, fractures may extend 

 much deeper into the earth than they would under normal conditions. 



a Russell, I. C. , Volcanoes of North America, Macmillan Co., New York, 1897, pp. 103-105. Also, 

 On the nature of igneous intrusions: Jour. Geol., vol. 4, 1896, p. 189. 



& Van Hise, C. E., Earth movements: Trans. Wisconsin Acad. Sci., Arts, and Letters, vol. 11, 1898, 

 pp. 493-494. 



