122 THE MAKQUETTE lEOX-BEARING DISTIUCT. 



hence these hitter rocks must he much okler thiui the detrital heds. In the 

 coiigh)meriites separating the granites from the overlying stratified beds 

 there are often fragments of schists, and fre(|uently there is a matrix com- 

 posed of comminuted greenstone. In the SW. \ of SE. J sec. 29, T. 48 N., 

 R. 25 W., a congh)merate composed of fragments of granite, quartz, and 

 green schist, cemented by a dark slaty material, lies immediately on the 

 contact of the greenstone-schist area with that of the detrital series. 



Sucli occurrences as these, when cousidered in coimectiou with the manner in 

 which the granite penetrates the greenish schists and is involved with them, seem to 

 render necessary the belief that, while it is plainly younger than the green-schists, it 

 is nevertheless greatly older than the overlying detrital rocks; and, more than this, 

 that when the latter rocks were spread, the granites and greenstone-schists together 

 had already suffered disturbance and deep denudation. It does not appear possible 

 to escape this conclusion by supposing that, since granite and greenstone-schists are 

 eruptives, they may have furnished fragments to almost contemporaneous sedimen- 

 tary deposits; for, in the first place, both the greenstone-schists and the gneissoid 

 granite must have received their schistosity before yielding the fragments. Moreover, 

 whatever may have been the depth at which the schistose rocks were first formed, 

 the granite masses which intruded them, according to all the later developments and 

 doctrines of petrography, must have been crystallized in depth, and must therefore 

 have had removed from over them great masses of materials before yielding fragments 

 to wave action. (P. 23.) 



These remarks refer to the coarse granites of the district, and not to 

 the fine-grained red granites which intrude the coarse granite and may 

 be even as young as the iron-bearing series, though none of its dikes have 

 been seen cutting the detrital rocks in the Marquette ai-ea. 



* * ' Accepting Professor Williams's conclusions as to the surface origin of 

 most of the greenstone-schists of the ^Nfarquette region, I should suppose that, after 

 the accumulation of these rocks to the thickness of several thousand feet, they were 

 intruded by granite bosses. These bosses perhaps may have been merely softened 

 portions of the underlying gneissic basement, which, indeed, maybe represented in an 

 unaltered condition in i)ortions of the granitic areas themselves, for all that has yet 

 been determined to the contrary. Subsequently mountain-making movements brought 

 about the folding and alteration of these enormous sheets of eruptive material, now 

 represented by the greenstone-schists. Following this was the great denudation 

 which brought to light the previously buried graniiic masses. This erosion was 



