ACID INTKUSIVBS. 195 



laere to repeat that they have been much mashed and recrystallized, but 

 that some of them still show their frag-mental origin. New biotite, musco- 

 vite, feldspar, and quartz have developed. Where most altered, the}^ are 

 mica-schists and mica-gneisses. These changes are presumed to be due, for 

 the most part, to the orogenic forces which were active prior to the intrusion 

 of the granite (p. 170). If such be the case, the granite began its metamor- 

 phic action upon a rock already greatly changed from its original condition. 



EVIDENCE OF INTRUSION. 



The intrusive character of this large mass of granite is indicated by 

 its stratigraphical position, and is further confirmed by the mechanical 

 effects produced by the intrusion of tke granite upon the sedimentaries, by 

 the contact effects produced in the sediments, and by the presence in the 

 sedimentaries of granite dikes forming offshoots from the main mass. 



The mechanical effects are well shown by the inclusion of sedimentary 

 fragments and by the dislocation and folding of the beds. Inclusions are 

 rather common and are usually of considerable size. 



The dislocation and folding are beautifully shown at N. 1970, W. 570 

 paces, sec. 30, T. 42 N., R. 3 1 W. In general the layers in the gray wacke, 

 which are alternately rich and poor in mica, strike N. 15° to 30° W., but 

 where the intrusives are, these layers are found to strike almost due north 

 and south. At the above location the beds are folded into small, closely- 

 compressed anticlines and syncliues, which plunge to the east at an angle 

 of about 80°. At this place the micaceous graywacke is broken into small 

 pieces, which are thoroug-hly injected and cemented by the granite, thus 

 forming a typical eruptive breccia. The granite cement is microgranitic, 

 with comparatively little quartz and a small amount of chloritized mica. 

 The fragments of micaceous graywacke in the breccia appear to be rather 

 more feldspathic than usual, but otiierwise seem not to have been much 

 affected. 



Owing to the altered condition of the sediments prior to the granite 

 intrusion, and to the alternation of sediments of somewhat varying char- 

 acter, we can not expect to find such clearly outlined concentric zones 

 surrounding the granite as in cases where the sediments are uniform. In 

 one case a contact was observed between the granites and apj^arentlv the 

 main mass of the sediments. Along this line of contact biotite and white 



