720 A TREATISE ON METAMORPHISM. 



One additional American illustration is given. Grant describes the 

 development of contact minerals adjacent to the great basal gabbro of 

 northeastern Minnesota.' 1 This is an interesting case, since the intrusive 

 rock is basic and since it is in contact with several different formations — 

 Animikie slates, Animikie iron-bearing member, green-schists, greenstones, 

 and granites. The prominent contact minerals developed in the slates are 

 feldspar, biotite, and muscovite, with occasional graphite, cordierite, and 

 hypersthene. The important contact minerals in the iron-bearing member 

 are magnetite, amphibole, augite, hypersthene, and olivine. In the meta- 

 morphosed greenstones augite and hypersthene are found in considerable 

 amounts. Besides the distinctive minerals developed, all of the rocks are 

 completely recrystallized for a distance varying from a hundred meters to 

 several hundred meters from the intrusive rock. 



Concluding this part of the subject, it is evident that the development 

 of the metamorphic minerals occurs largely while intrusion is taking jdace 

 and movement is still going on, which explains the marked tendency for 

 the development of slates, schists, and gneisses. Following the intrusions 

 of the great batholiths and the orogenic movements connected with them 

 the mass conditions are static, yet the conditions may be very favorable 

 for recrystallization. The temperature may still be high and the solutions 

 active, and under such circumstances the conditions are very favorable for 

 the development of porphyritic crystals which have no relation in their ori- 

 entation either to the batholiths or to the slates, schists, or gneisses. Hence 

 it follows that within these rocks peripheral to the batholiths we find the best 

 illustrations of secondary porphyritic crystals. Moreover, the porphyritic 

 crystals which appear are dependent upon the intensity of the metamor- 

 phism. The order of the heavy porphyritic crystals in relation to the bath- 

 oliths is that above mentioned, viz., near the batholiths are apt to be the 

 heavy silicates, tourmaline, staurolice, and cyanite; more remote from them, 

 g'arnet and sillimanite; and still more remote, andalusite. Of course these 

 overlap and are intermingled, as alreadj^ explained. 



PEGMATITES. 



The complex processes in connection with the intrusive batholiths 

 and their offshoots may, and indeed do, produce masses of rocks which 



"Grant, V. S., Contact metainorphism of basic igneous rock: Bull. Geol. Soc. America, vol. 11, 

 1900, pp. 503-510. 



