178 THE MARQUETTE lEON-BEAKING DISTKICT. 



THE INTRUSIVES IN THE NORTHERN COMPLEX. 



The granites, gneisses, and schists of the Northern Complex are cut by 

 numerous dikes of basic and acid material and certain boss-like masses of 

 peridotite, or of its altered form, seqjeutine. Of the dikes the basic ones 

 are much more common than the acid ones, if we exclude from the latter 

 those that are but apojthvses of the coarse granite. 



THE BASIC DIKES. 



The basic dikes cut the gneissoid granite, the syenite, and the schists 

 indiscriminately, though they may be most abundant in the greenstone- 

 scliist areas. They vary in width from an inch or two to 75 feet or more, 

 and some of them have been followed 2 or 3 miles. 



These dikes have been so well described by Williams' that there is 

 little left to be said in this place concerning them. Diabases, epidiorites, 

 and diorites were distinguished by this author. The diabases are of the 

 usual types. The epidiorites are thought to be uralitized and epidotized 

 diabases, since their structure is plainly ophitic, the feldspar occurring 

 in lath-shaped crystals, and the amphibole forming fibrous wedge-shaped 

 masses between the plagioclase laths. The diorites differ from the epidiorites 

 mainly in structure and in the nature of their horublendic component. The 

 amphibole in the diorites is compact and idiomorphic, and hence it was 

 considered by Williams as original. In some slides of these rocks, how- 

 ever, are cross-sections of a compact brownish-green hornblende that is 

 perfectly idi<imorphir, while at the same time nests of light-colored augite 

 may be seen included within its mass. If this hornblende is secondary, 

 as it seems to be, then it is probable that many of the supposed diorites of 

 this district are altered dialjases, just as are the epidiorites, which contain 

 fibrous amphibole. 



The freshest dialjases, those still containing large quantities of pyroxene, 

 are quite massive, even when the rocks through wliicli they cut are com- 

 pletely schistose. These, then, must have been intruded in the schists after 



' The greeustone-schist areas of the Menominee ami Marquette regions of Michigan, by G. H. 

 Williams: Bull. U. S. Geol. Survey Xo. G2, 1890, pp. I38-14B, 1(58-175, 180-184, 189-190. 



