General considerations.) 



MINERALOGY AND PETROLOGY. 



985 



ophitic structure it has the evident granular condition of muscovadyte. But, so far 

 as the second section shows (it is a larger one), the contained minerals are almost 

 exclusively labradorite and diallage, making the rock a normal and typical gabbro. 



FIU. 50. GABBKO ON "MUSCOVADO" FRAGMENTS. 



When it is understood that this rock at this locality is recognized as a part of the 

 metamorphosed Keewatin of the region, and is seen in great amount along the 

 shore of the island, and widely on the mainland, where (as has already been stated) 

 it is traceable directly into the greenstones, it seems to afford unimpeachable testi- 

 mony, concise and direct, of the change from the greenstones to the gabbro. The 

 facts of nonconformity and intrusive action are only accidents of an epoch of general 

 metaniorphism and refusion, and hence do not necessarily separate the intruded 

 rock from the intrusive by any break in genetic relationship. 



3. (ieneral considerations. If these rocks be not thus genetically united, and 

 derived from the Keewatin, there is no possible way known to account for their 

 variations. They include not only the basic rocks that are distinctively greenstone 

 or diabase and gabbro, but more acid phases. Quartz occurs in the gabbro sporad- 

 ically, likewise orthoclase. In some places hornblende replaces augite, etc., etc. 

 The names that have been given to these and other variations are numerous (see the 

 discussion in Part I), but these various rocks, both geographically and petrograph- 

 ically, shade into each other and into normal gabbro. In order to explain these 

 shades of variation on the hypothesis of differentiation from an intermediate or other 

 primordial magma the different stages of change must have been lost in each other, 

 and, furthermore, they must be allowed to have been cotemporary in the same magma 

 and in opposite directions. This would require such uniformity, and at the same time 

 such variety of primordial magmatic conditions in the same magma that the idea 

 becomes self-contradictory and impossible of rational application. If these variations 

 be supposed to be due to separate intrusion after progressive differentiation had 

 supplied different types of rock for such a result, then those types would not, at the 

 surfaces blend into each other, but one would intrude into the other and show a 

 sharp contrast or contact with it. The general trend of all the facts, both petro- 



