508 U. S. GRANT — CONTACT METAMORPHISM OF BASIC IGNEOUS ROCK 



olivine, augite, and hypersthene. To present this point more definitely, 

 it can be stated, first, that from the quartz and magnetite of the slates 

 can be formed the quartz and magnetite of the metamorphic rocks; 

 second, that from the actinolite and hornblende of the slates can be 

 formed the augite and the hornblende of the metamorphic rocks ; third, 

 that from the griinerite (FeSiO :J ) can be formed the fayalite (Fe 2 Si0 4 ) ; 

 fourth, that from the cummingtonite ([MgFe]SiO :} ) can be formed the 

 olivine ([MgFe] 2 SiOJ and the hypersthene ([FeMg]Si0 3 ). It is thus 

 clear that the materials necessary for the formation of the minerals of 

 the metamorphic rocks were all present in the original quartz-magnetite- 

 amphibole slates. It is not the intention of the writer to state that there 

 was no transfer of material from the gabbro during this metamorphism. 

 This may have taken place, but it is not necessary to assume even a 

 limited transfer, for the materials requisite for the manufacture of the 

 minerals of the metamorphosed rocks already existed in the iron-bear- 

 ing slates. 



Turning now to the reasons for regarding these peculiar metamorphic 

 rocks as not facies of the gabbro, it may be stated : 



1. That the rocks in question contain no feldspar, which is the most 

 abundant mineral in the gabbro. 



2. That quartz is abundant in the metamorphic rocks, while it is absent 

 in the normal gabbro. If these rocks were facies of the gabbro we would 

 have the anomalous feature of much free quartz in an ultra-basic rock. 



3. That the magnetite of the olivinitic iron ores is not titaniferous, 

 while the magnetite masses of the gabbro are highly titaniferous, and 

 the magnetite of the ordinary gabbro is also titaniferous. 



4. That where the two rocks come into contact there is no gradation 

 between them, but a sharp and definite separation can be made. More- 

 over, at the contact the gabbro is finer grained than away from this line. 

 The same sharp separation can be made where sills from the gabbro lie 

 in the iron-bearing rocks, and here the fineness of grain of the sills at 

 the contact is very marked. 



5. That there are nowhere in the gabbro such intimate laminations 

 and bandings as in the rocks in question. 



These iron-bearing rocks have suffered much more extensive recrys- 

 tallization than other strata with which the gabbro has come in contact. 

 The. exact reason for this is not clear, but probably the peculiar compo- 

 sition of these rocks in part conditioned the result. 



THE KEEWAT1N 



Where the gabbro is in contact with strata of Lower Huronian age 

 there has been the usual recrystallization, but as these strata are diverse 



