MUSEUM OF COMPARATIVE ZOOLOGY. 53 



the actinolite schists of Wichmann (100, p. 47), Mr. Brooks believes* it 

 to be " Laui'entian," for no reason that we can see except that it is associ- 

 ated with the granite. This rock has been tilted and contorted by the 

 granite which is found in contact with it. The line of junction and the 

 manner of contact show that the granite was the later rock and an erup- 

 tive one. The foliation of the granite is parallel to the plane of contact, 

 or at right angles to the pressure. It contains fragments of schist (98), 

 while some highly quartzose schists or quartzites (99) were seen within a 

 few feet of it. These rocks were apparently older than the granites, and 

 had been affected by them. On the southwest side of Republic Mountain 

 the granite was seen about one rod from the " Huronian " schists. This 

 is probably the locality figured by Mr. Brooks,t but the foliation of both 

 appeared to us to be conformable in this way : the schists appeared to 

 have been partially uptilted by the granite, which seemed to have been 

 extravasated obliquely out from under them, very much as the perido- 

 tite was under the sandstone at Presque Isle. The foliation is parallel 

 to the plane of pressure, and at right angles to that pressure. 



The granite near the edge is fine-grained, resembling a quartzite 

 (128), but a little distance away it is coarser and more granitoid (129). 

 Borne upon the face of the granite next to the schists is a plate of rock 

 about two inches thick (130). This is welded closely to the granite, 

 and has been uplifted and altered by it. Its constituents have been 

 drawn out in the direction of the supposed motion of the granite, and re- 

 semble in the field the altered garnetiferous "diorite" (123) described on 

 page 47. The rock is micaceous, of a dark gray color, and contains elon- 

 gated brownish-gray masses resembling altered garnets. Microscopically 

 it is seen to be composed of biotite, muscovite, quartz, and the garnet(?) 

 masses. The last are now composed of a finely fibrous aggregately po- 

 larizing material holding quartz grains, magnetite, and apparently hex- 

 agonal or orthorhombic opaque disks. Their nature is unknown, but 

 they most probably belong to the margarophyllites.l Whether this 

 rock was originally the same as No. 123 camiot be told, although in 

 many respects it closely resembles it. Should it be, the granite there 

 is younger than the " Huronian diorite." 



The granite (129) is seen in the thin section to be composed of quartz 

 with some green mica. Not the slightest ti-ace of feldspar was found. 

 The quartz is broken up into grains, which exactly fit to one another 

 without any cementing material. The granular structure arises not 



* Geol. of Wise, III. 661. | Am. Jour. Sci., (3,) X. 20. 



t Geol. of Mich., I. 126. 



