PEBVIOUS WORK. 



15 



This rock is composed of mica, quartz, and feldspar. Its laminas are undulating 

 or waved, but its cleavages, on a large scale, are even and regular 



These mica-slates are best developed on the south boundary of township 42 N., 

 ranges 31 and 32 W., in the beds and banks of the Peshakumme and Mesqua-cum-a- 



FiG. 1.— Reproduction of a portion of the geological map of the Upper Peninsula of Michigan by Wm. A. Burt, 1846. 



cum-sepe, and at the falls near the junction of the latter stream with the Brula River. 

 [These, according to our observations, are the most altered phases of the Upper 

 Huronian.] 



That part of the geological map accompanying the above report which 

 corresponds to the Crystal Falls district is reproduced in fig. 1. 



