CHAPTER Y. 

 THE IGNEOUS ROCKS. 



By W. S. Bayley. 



The igneous rocks associated with the Marquette sediments and ores, 

 while varying in their present character, were originally nearly unifonu in 

 their mineralogical composition. Although occurring as bosses, dikes, inter- 

 leaved sheets, surface flows, and tuffaceous beds, which have suffered a 

 greater or less amount of alteration into ])roducts which are now not a little 

 unlike one another, they Avere all, so far as has been determined, originally 

 basic rocks of the composition of diabases. The variety at present exhibited 

 by them is due almost exclusively to subsequent alterations. 



Most of the rocks here considered, including the "diorites," "diorite- 

 schists," "chlorite-schists," "magnesian-schists," "soapstones," and "paint- 

 rocks," have been regarded by some geologists as metamorphosed sedi- 

 mentary fragmentals. As we shall discover later, there is not a particle of 

 evidence for this assumption. Even the most schistose of these rocks, with 

 the possible exception of some of the "soapstones" and "paint-rocks," are 

 certainly of igneous origin. The pyroclastic beds, so abundantly developed 

 in the western portion of the district, and constituting a large portion of the 

 Clarksl)urg formation of the Upper Marquette series, are, of course, frag- 

 mental, but they are of volcanic and not of sedimentary origin. 



For convenience of discussion the igneous rocks are separated into two 

 classes, in the first of which are placed those associated exclusively with the 

 beds below the Clarksburg formation, and in the other those cutting also 

 the beds above this terrane. The latter are evidently younger than the 



