IGNEOUS EOCKS OF STURGEON RIVER TONGUE. 487 



around them exactly as tliey would do were the large hornblendes present 

 before the rock was squeezed. Sometimes the amphibole masses that form 

 so large a proportion of the schistose bands are single crystals, sometimes 

 they are fragments of crystals, and at other times they are groups of 

 radiating crystals. The magnetite is very much more abundant in the 

 hornblendes than in the surrounding quartz aggregate, sometimes being 

 confined exclusivel}' to this mineral, as though it were one of the products 

 (the hornblende being the other) resulting from the decomposition of some 

 original constituent, probably augite. Little particles of hematite, on the 

 other hand, are abundantly disseminated through the quartzose aggregate, 

 and are practically absent from the hornblende. Much of it appears to 

 have been derived from magnetite. 



The evidence derived from the microscopic study of sections of these 

 banded rocks, so far as it relates to their crigin, is disappointing. The 

 quartzose layers are, in all probability, sedimentarj-. The hornljlendic 

 layers, however, differ from these so much in composition that their material 

 must have had a different source. It is possible that the quartzose layers 

 rejjresent sediments derived from the granitic portions of the Basement 

 Complex, while the hornblendic layers represent sediments derived from 

 the basic portions of the Basement Complex; or, it may be that the acid 

 layers have the origin ascribed to them, Avhile the basic ones are mixed 

 sediments and basic tuffs. The sections of the dark layers of these rocks 

 resemble so strongly the sections of the basic layers in the Clarksburg series 

 of mixed tuffs and sediments in the Marquette district that the writer is 

 inclined to regard the rocks as composed partly of tuifaceous material. On 

 the other hand, the banded rocks occur so close to the boundary between 

 the sedimentary area and the Basement Complex, which near this boundary 

 is composed mainly of basic schists, that it would seem but natural that 

 they should contain large quantities of basic material derived from these 

 schists. The original structure of the layers has been so completely 

 destroyed by mashing that it can not give any evidence as to the natiu-e 

 of the beds. We are therefore compelled to rely entirel}^ upon their com- 

 position to aid us in discovering their origin. This indicates simply that 

 much of their material was derived either from volcanic ashes or from the 

 debris washed from the basic portions of the Basement Complex. 



