204 THE MARQUETTE IRON-BEARING DISTRICT. 



seems to be every .stag-e of gradation. In tlie one class are placed tlie 

 greenstoiie-scliists, composed of hornblende, plagioclase, and the alteration 

 products of the feldspar, and in the other class a series of lustrous, black, 

 foliated rocks, which we shall call ampliil)ole-schists. They consist essen- 

 tially of green hornl^lende, fresh plagioclase, aiid quartz. All of these rocks 

 are so similar to certain phases of the green schists of the Northern Complex 

 that their descriptions need not detain us long. The greenstone-schists are, 

 clearly, altered and foliated basic crystalline eruptives, and since they pass 

 bv intermediate phases into the amphibole-schists, it is believed that these 

 also are squeezed erupti^'es, in spite of the fact that they are often banded 

 and that some of thejn contain no inconsiderable quantity of quartz. 



liUEKXSTONK-SClIISTS. 



The greenstone-schists in the hand specimen and in thin sections 

 resemble more closely those schists of the Northern Complex that were 

 derived fi-om basic dikes and lava flows than they do those derived from 

 tuffs. In the hand specimen they present a wide variation in appearance. 

 Some of them are fine-grained, light greenish-gray, almost massive, or 

 slightly foliated rocks ; others are dark-gray, fibrous schists ; while still 

 others are finely banded, green and white schists. The latter are less 

 common than the other two varieties. 



In the thin section nearly all the rocks show plainly their original 

 character. Altered plagioclase and green hornblende are their principal 

 components. The feldspar is changed more or less completely into an 

 aggregate of epidote, saussurite, quartz, and chlorite, with occasionally a 

 small admixture of a micaceous mineral. In addition to the altered plagio- 

 clase there is also present in many sections a fine mosaic of fresh 

 plagioclase, resembling the feldspathic mosaic of many of the greenstone- 

 schists of the Northern Complex. An untwinned decomposed feldspar, 

 which is thought to be orthoclase, is also met with in a few sections. Its 

 alteration products are mainly sericite or muscovite. The green amphibole 

 is in three forms ; it exists as long, slender needles penetrating the decom- 

 position aggregates of the feldspars, as large plates and aggregates of flakes 

 occupying spaces formerly occupied by augite, and as compact crvstals, idio- 

 morphic in the prismatic zone. The abundance of the compact aui2)hibole 



