A STUDY OF SOME EXAMPLES OF ROCK 

 VARIATION. 1 



[Published by permission of the Director of the United States Geological Survey.] 



In the following article I purpose to describe briefly a series 

 of igneous rocks which vary from those of acid character through 

 those of intermediate and basic characters to others which are 

 ultrabasic in composition. The rocks occur upon the upper pen- 

 insula of Michigan, in the vicinity of Crystal Falls, the most 

 important town in the iron-bearing district of the same name. 

 All are to be found in an area extending from Crystal Falls 

 southeast to a mile east of the Michigamme River. In the des- 

 cription of the various kinds no detailed mention of localities 

 will be given, since they are given in the complete article, and 

 if wanted can readily be found by reference to it. The rocks 

 occur as knobs or groups of knobs, and as well determinable 

 dikes cutting the knobs. The outcrops project through an area 

 covered by glacial deposits. The best exposures are naturally 

 near the river, where erosion has removed the drift mantle. 



Here and there in the drift are isolated exposures of sedimen- 

 tary rocks of Upper Huronian age. The relations of the igneous 

 rocks to the sedimentaries are not shown by exposures of direct 

 contacts, but it is inferred from the occurrence of sedimentaries 

 between the igneous exposures that the drift is underlain by 

 Upper Huronian sedimentaries, and that the igneous rocks are 

 intrusive in them. The intrusives have never been found to 

 penetrate the superimposed, horizontal, Lake Superior, Potsdam 

 sandstone. These facts are conclusive proof that their period of 



1 This article is a brief abstract of a part of a report which will appear in full as a 

 monograph of the U. S. Geol. Surv., under the title of "The Crystal Falls Iron- 

 bearing District of Michigan." For details which are not warranted in this place the 

 reader is referred to the monograph. An abstract of the monograph, in which the 

 portion treated in the present article is barely mentioned on account of lack of space, 

 will be published under the same title as the monograph in the 19th Annual Report. 



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