206 THE MARQUETTE IROX-BEAKING DISTRICT. 



be described presently. Tlie rocks of tliis tx-pe present a very fresh aspect. 

 As a rule no alteration products can be detected in them. The rocks are 

 now composed of clear jjlagioclase and dark-g-reen amphibole, and usually 

 some biotite. The plagioclase is in mediuiuly coarse grains that interlock 

 in the manner of dioritic plag-ioclase. These are often dusty, with small 

 inclusions of magnetite, amphilxile, etc. The amphibole is in large plates, 

 often twinned, and nearly alwa^'s idionuirijliic in the prismatic zone. The 

 mineral is cellular, ])ossessing the structure of Salomon's contact minerals, 

 and it is in its present form younger than tlie feldspar. Tlie biotite is of 

 the usual reddish-brown color. It occurs in small flakes that lie between 

 the plagioclases. 



In structui-e and composition these rocks are "diorites," like those 

 described by Williams^ from the Northern Complex, but they are believed 

 to be altered basic rocks. In some eases, wdieu the gi-aiu is a little finer than 

 in the type described above, the origin of the rock is fairlj^ well indicated. 

 In addition to the components mentioned, there are often large grains of a 

 decomposed feldspar in the midst of a mosaic of fresher ones. Tlu- former 

 are clouded with small biotite flakes and small grains of quartz, and are 

 bordered by a clear mosaic of plagioclase. In the more massive forms of 

 the rocks the outlines of the original large grains may be detected. In the 

 schistose phases these gradually disappear as the schistosity becomes more 

 marked, until in the highly foliated phases all trace of the large (doudy 

 grains disappears, and the rocks now are aggTegates of green hornblende in 

 a mosaic of clear jdagioclase g-raius and brown biotite flakes. The coarser 

 schists are believed to have the same origin as those in which the jdagio- 

 clase is in the form of a fine-grained mosaic, the only difference between 

 the two rocks being in the size of the grains of the secondary plagioclase. 

 Both are believed to be dynamically metamorphosed forms of a basic 

 intrusive rock which may have been a diorite, a gabbro, or a coarse diabase. 



VMI'IIIIiOLi; 



The amphibole-schists are distinguished from the greenstone-schists by 

 the possession of ([uartz. This mineral is sometimes present in very small 



' The greeustone-scUist areas of the Meuoniinee auil Mar(iuette regions of Michigau, hy G. H. 

 Williams: Bull. U. S. Gcol. Survey No. 62, 1890, p. 146. 



