500 THE MAEQUETTE IKOXBEARING DISTllICT. 



T. 48 N., R. 30 W. (Atlas Sheet V); further, numerous inclusions of 

 the gTiinerite-magnetite-schists are found at one place within the g-reeu- 

 stone (PI. XII). 



With respect to the Republic greenstones, Rominger' well says: 



The whole slope of the hillside is composed of an endless succession of banded 

 ferrngino-siliceous, actinolite schists, united into bulky, compact masses, which are here 

 and there interrupted by intrusive diorite belts of short, local exteusion, and not, as 

 represented by Major Brooks, in regular continuous bands encircling the whole side 

 of the mountain. 



Although it is clear that all the greenstones of the western knobs and 

 those of Republic Mountain are intrusive, it is not known in all cases 

 whether they possess the features of dikes, sheets, or bosses. In some cases, 

 as at the Spurr mine, the greenstones are apparently interbedded with the 

 sedimentary formations ; but even here the supjjosed beds are of short linear 

 extent, and may be dikes that happen to coincide in direction with the 

 strike of the bedded rocks at the position of the present plane of erosion. 



From the general relations of the western greenstones, exclusive of the 

 smaller dikes, it would appear that most of them are boss-like dikes whose 

 courses on the surface are approximately parallel to the strike of the sedi- 

 mentary beds intruded by them. • A few may be in the form of sheets, but 

 if so they are intrusive and not surface sheets, and hence are not available 

 for working out the structure of the Marquette series. They do not con- 

 stitute well-detined beds at definite horizons in the series, as was supposed 

 by Brooks. 



PKTIiOGRAPHICAI. CHARACTER. 



The rocks of the western knobs differ materially from those of the 

 eastern knobs in their microscopic features, although some of them are A-ery 

 like the latter macroscopically. Originally there was probably no difference 

 between the two types of rock. In the eastern greenstones, however, the 

 alteration was degradational in its nature. The diabases have passed 

 through epidiorites and chlorite-schists into aggregates of calcite, chlorite, 

 epidote, etc., all of which may be regarded as final stages in the weathering 

 of plagioclase and pyroxene. In the western greenstones dynamic processes 



' Geological Survey of Michigan, Vol. IV, p. 101. 



