710 A TREATISE ON METAMORPHISM. 



(See pp. 658-658.) I have said that it is characteristic of the zone of 

 flowage that no large openings can be supposed to exist continuously. It 

 does not therefore follow that at times of rapid movement extensive fractures 

 may not be temporarily formed. If these were at a place of subtuberant 

 intrusion, or were adjacent to any magma, material would break away from 

 the central mass, intrude itself into the opening's and fractures, wedge the 

 walls apart, and thus we should have intrusives for a considerable belt 

 peripheral to the central mass which are analogous in their forms to the 

 intrusives characteristic of the belt of cementation — that is, intrusives along 

 fractures; and this would be so even if the rock alterations of the place 

 were, on the average, those characteristic of the zone of anamorphism 

 rather than of the zone of katamorphism. We have here really the case of 

 an intermediate belt of combined fracture and flowage, fractures extending 

 deep at times of rapid deformation, although the normal conditions are 

 those of flowage. Thus from the great central masses of igneous rocks 

 smaller masses extend in various directions to various distances, some being 

 subordinate subtuberant masses, and others taking advantage of the tem- 

 porary fractures and thus forming dikes There is a marked tendency for 

 these subordinate masses to follow planes of weakness, as, for instance, the 

 contacts between rocks of different character, the bedding of sedimentary 

 rocks, or other structures such as the planes of cleavage in schistose rocks. 

 Intrusives of this kind usually have greater dimensions parallel to the 

 structures followed than in the transverse directions. The tendency of 

 the injections to follow cleavage is very marked indeed. Parallel to the 

 planes of cleavage in the slaty and schistose rocks intrusions may be very 

 close together and small, there being perhaps a considerable number within 

 the breadth of an inch; or the intrusives along cleavage planes may be 

 larger and far apart, or they may be any combination of these. All these 

 phenomena are well illustrated by the Hudson schist of New York City, 

 especially at New Rochelle, and by the gneisses at many places upon the 

 Piedmont Plateau. 



(3) Finally, the upward progress of the great masses of magma is also 

 doubtless made in part by the fusion and absorption of the material with 

 which it is in contact, although the evidence that such absorption has taken 

 place on a great scale is lacking. That absorption does take place in a minor 

 way, however, is certain. (See "Fusion and absorption," pp. 728-736.) 



