410 THE PEifOKEE lEON-BEAEING SERIES. 



SECTION v.— THE GREENSTONES. 



The eruptives belonging to the Eastern area may be considered in 

 three divisions, l>ased upon apparent geographic continnity or hthologic 

 likeness, or botli. The first division includes the very numerous exposures 

 of greenstone which outcrop in Sees. 15, 16, 23, 25, 26, and 27 (except 

 the northern exposure in Sec. 15), T. 47 N., R. 44 W., Michigan. The 

 second area is in Sees. 20, 29, and 30, T. 47 N., R. 43 W., Michigan, and 

 the third is in Sees. 13, 14, 15, and 24, T. 47 N., R. 44 W., Michigar.. 



The main area. — The exposures in Sees. 15 and 16, and those in 

 Sees.' 26 and 27, belonging to this area, are of large size. The greenstone 

 iii Sees. 15 and 16 forms a continuous higli range, running east and west, 

 au'd apparently joins the great greenstone ridge to the north and west 

 belonging to the Copper-bearing series. In traversing the ridge in a 

 north and south direction one has to ascend a height of some 500 feet above 

 tlie valleys to the north and south. Also the large exposure near the center 

 • of Sec. 26 is another notable landmark, rising to about the same eknation 

 above the level country surrounding. 



Lithologically, the exposures in tliis area are essentially the same 

 rock. The greater number of them are diabases, but they grade into 

 gabbros. Tlie gabbros have a granitic structure, and the pyroxene shows 

 the diallagic cleavage. The contrast between the structure of these gabbros 

 and the diabases with perfect ophitic structure, }>oth belonging to the same 

 area and probably to the same rock mass, is very great. One of the 

 exposures in Sec. 25 has very large augites, each of which includes scores, 

 if not hundreds, of small lath-shaped plagioclases. The gabbros are found 

 in the central and western parts of the areas, while it is further notable 

 that the rocks in the west part of Sec. 23 and near the center of Sec. 15, 

 that is, tliose which are nearest to the greenstone-conglomerate area, are 

 tlie ones which become fine grained, and the typical ophitic structure 

 a])peai's. And in the exposure near the center of Sec. 15 an amygdaloidal 

 or pseudo-amygdaloidal appearance is presented. It does not, then, ap])ear 

 improbable that these greenstone masses are or were really connected witli 

 tlie greenstone-conglomerates, the latter perhaps being the siiiface material, 

 while tlic (liabas(^s and gabbros were dec]) seated. As these diabases are 



