136 THE VERMILION IRON-BEARING DISTRICT. 



The isolation of tlie belts is due to the effect of the cross folding which has 

 produced anticlines of greenstone plunging down under sediments that 

 wrap around them. The most striking cases of these isolated anticlines are 

 those shown by the distribution of the greenstone in the vicinity of Knife 

 and Cacaquabic lakes and between Ogishke Muncie and Grobbemichigamma 

 lakes. Looking at the distribution of the greenstone proper, we see that 

 the presence of the synclinal structure is most marked in the western part 

 of the district where the younger formations are infolded into the greenstone 

 and where the greenstone predominates. In the eastern part of the district, 

 on the other hand, the anticlinal structure of the greenstone is most marked 

 for the reason that the minor synclines are very deeply buried by the 

 sedimentaries, which have a great surficial extent, as a result of which only 

 the crests of the anticlines are exposed where they project through the 

 sedimentaries. 



PETROGRAPHIC CHARACTERS. 



The rocks comprised in the Ely greenstone originally corresponded in 

 character to intermediate andesites and basic basalts and, like the recent 

 representatives of these families, must have been black, or dark gray, and 

 presumably likewise corresponded to them in mineralogic character. These 

 Archean rocks have undergone for so long a period the vicissitudes to 

 which all rocks are exposed that it is not to be wondered at that they have 

 for the most part been exceedingly altered and never show all of their 

 original characters. Indeed, it is surprising that they retain any of their 

 original structures. Most of the changes which have affected them have 

 been in the character of the minerals, and these will be described in the 

 proper place. The changes that are most obvious macroscopically are 

 the chemical ones which have affected their color and the mechanical ones 

 which have affected their structure. 



While the rocks, on the whole, are of a greenish color (hence the 

 general name of greenstone), the various phases of them show all possible 

 varia,nts of this, ranging from very light-colored greenish gray to very 

 dark gi-eenish black, with the light grayish and brownish greens 

 predominating. The rocks appear lighter on the weathered surface, as a 

 rule, than upon fresh fracture surfaces. Some of the greenstones are very 

 much more • feldspathic than usual, and in such cases they weather with a 

 light-pinkish crust, which causes them not uncommonly to be mistaken. 



