188 THE GKYSTAL FALLS lEON-BEAEING DISTRICT. 



ORDER OF TREATMENT. 



For the sake of convenience and t(:) avoid repetition, the ag-e of the 

 intrusives as a whole is first determined, and tlien follows a bi-ief descrip- 

 tion of the effect of the folding of the district on the distribution of the 

 eruptives. 



The rocks described have been divided into those of acid, basic, and 

 ultrabasic composition, and the usual order of discussion from the acid to 

 the basic will be followed. In Section I rocks are described which are 

 geologically disconnected. In Section II is a description of a series of 

 rocks which constitute a geological unit, and are especially interesting from 

 a petrogenetic standpoint. Here also the order of discussion is from the 

 acid to the basic. The geographical distribution of the rock of each divi- 

 sion is given, only those outcrops being accurately located which are of 

 very large size or which for other reasons are of special interest from a 

 stratigraphical or a jjetrological standpoint. 



After the petrographical description of each of the rocks a statement 

 will be made of its field relation to the adjacent rocks, if its relations haA^e 

 been discovered. This will be followed by a brief account, in those rare 

 cases in which a contact has been observed, of the metamorphic action 

 which it has caused in the sediments through which it was forced. 



AGE OF THE INTRUSIVES. 



The intrusives have forced their way through the Lower and Upper 

 Huronian sedimentaries, but have never been found to penetrate the hori- 

 zontal Lake Superior (Cambrian) sandstone. These facts alone are con- 

 clusive proof that their period of intrusion falls in the time which elapsed 

 between the deposition of the Upper Huronian and that of the Cambrian. 



In the discussion of the time of the folding of the Upper Huronian the 

 conclusion was reached that this folding preceded the deposition of the 

 Keweenawan series. If the intrusives to be described had existed at the time 

 of folding, the}' naust certainly have suffered from the orogenic movements. 

 Examination of the exposures of the intrusives has not shown schistose 

 masses, nor has detailed microscopical study disclosed the cataclastic textures 

 which accompany powerful dynamic movements, except in one case, which 

 is described on p. 1 94, and is presumed to be due to purely local movements. 

 Such being the facts, the conclusion follows that the intrusi^'es were intro- 



