SOME EXAMPLES OF ROCK VARIATION 373 



intrusion fell in the time between the deposits of the Upper 

 Huronian and those of the Cambrian. In the complete article 

 there is a discussion of the time of the folding of the Upper 

 Huronian, and the conclusion is reached that this folding took 

 place immediately preceding the deposit of the Keweenawan 

 series. If these intrusives had existed at that time, they must 

 certainly have suffered from the orogenic movements. Examina- 

 tion of the exposures of the intrusives has not shown schistose 

 masses, nor has detailed microscopical study disclosed any tex- 

 tures which accompany powerful dynamic movements, except in 

 isolated cases which are presumed to be due to purely local 

 movements. Such being the case, the conclusion follows that 

 these intrusives are subsequent in their origin to the folding of 

 the Upper Huronian ; that is, are of Keweenawan or of post- 

 Keweenawan age. 



It seems highly probable, though it cannot now be proven, that 

 the intrusives are contemporaneous with the period of volcanic 

 activity, during which the heterogenous Keweenawan series was 

 formed. During the formation of such a great series we might 

 well expect more or less Assuring of the sedimentaries and 

 intrusion of molten magma in a district no farther removed 

 from the scene of eruptive activity than is the Crystal Falls 

 district from the Keweenawan, a distance of about thirty miles. 

 It is at least clear that the rocks are post-Huronian and pre- 

 Potsdam, but a closer approximation cannot be made with 

 certainty. 



With few exceptions the intrusive rocks are medium to 

 coarse-grained. While the granitic texture is unquestionably 

 predominant, other textures are not absent, for we find some 

 ophitic and porphyritic textured rocks, and others in which 

 even a parallel (flow) texture has been produced. It has 

 already been intimated that the chemical range is very great. 

 There is, of course, a corresponding range in color. 



The main classes with which the variations will be grouped 

 are the diorites, the gabbros, or gabbro-norites, and the perido- 

 tites. Complete and accurate chemical analyses of certain 



