MUSEUM OF COMPARATIVE ZOOLOGY. 75 



buried under later formations, yet in themselves we consider that they 

 give no evidence of it. 



The general structure of the country would seem to be as follows. 

 The schists, sandstones, etc., having been laid down in the usual way, 

 were then disturbed by the eruption of the jasper and ore ; this 

 formed the knobs of jasper, the banding belonging to the fluidal struc- 

 ture, and not to sedimentation. Besides occurring in bosses, the jasper 

 was spread out in sheets, and intruded through the rock in wedge- 

 shaped masses, sheets, and dikes. Much of the original rock still re- 

 mained horizontal, and new sedimentary deposits continued to be formed 

 out of the jasper and the other rocks. Ne.xt came the eruption of " dio- 

 rite," which completed most of the local folding and tilting of the strata. 

 Finally, the granite eruption took place on both sides of the " Huronian," 

 uplifting and contorting the strata near it, and perhaps laterally com- 

 pressing the enclosed iron-bearing rocks. No basis e.xists so far, then, for 

 the scheme of formations laid down by Mr. Brooks, as it was founded ou 

 the supposition that all the rocks were sedimentary. The other results 

 of our work are : the showing more clearly the age, origin, and nature 

 of the peridotite at Presque Isle, and the formation of serpentine from 

 it t?i s^V«; the finding of ottrelite schist as a metamorphosed rock from 

 the ordinary schist ; the showing that tourmaline and olivine are more 

 abundant here than heretofore known ; the finding of granite and fel- 

 site within the Marquette district, etc., etc. 



Concerning the serpentine (peridotite) of Presque Isle Mr. Brooks re- 

 marks, " It is probably Huronian, and presents some of the phenomena 

 of an eruptive mass." * Later he says, "It is not certain that this is of 

 Huronian age." t 



Although in deference to the common custom we have employed the 

 term jasper in writing of the silicious eruptive rocks associated with tlie 

 ore, in reality it is not properly called so. It is often uncolored, and 

 has then generally been designated by observers as a quartz schist, they 

 believing it to be sedimentary. The rock further has been denominated 

 felsite or petrosilex, but physical and chemical charactei-s remove it fi'om 

 these old rhyolitic rocks. It is more acidic than the rhyolites, the silica 

 being above eighty per cent. We also found other eruptive rocks of like 

 acidic character, and, so far as our observations have now gone, it seems 

 probable that rocks of this class are much more abundant than we should 

 at first suppose. They would naturally be taken at first sight for 

 quartz schists, quartzites, and other sedimentary rocks, and no further 

 * Geol. of Wise, III. 532. t Ibid., 659. 



