THE LOWER HURONIAN. 343 



Some of tlie best places at which to study the slates that have been 

 extremely metamorphosed are on the east, southeast, and south shores of 

 Gobbemichigamma Lake. Here the metamorphism of the slates by the 

 gabbro has not been complicated by a previous metamorphism of the slates 

 by a granite, as is the case in the vicinity of Snowbank Lake. At various 

 places here the sediments are exposed, showing' in places their characteristic 

 banding, but they have been so extremely altered that but for this banding 

 their derivation from the slates might not be recognized. The sediments 

 have acquired for the most part a granular character and brownish coloi', 

 and weather rather readily. As the result of their peculiar saccharoidal 

 appearance the name "muscovado," having reference to their resemblance to 

 brown sugar, was given to them by Alexander Winchell." Their true char- 

 acter was not recognized by him. Since it is clear that they are but meta- 

 morphosed phases of the sediments, it seems totally unnecessary to continue 

 the use of this term, which can not be applied to a rock of definite composi 

 tion, especially since, as pointed out by H. V. WinchelP and U. S. Grant," 

 two different kinds of rocks, metamorphosed sediments, and certain phases 

 of the gabbros, are included under this term merely because they bear a 

 superficial resemblance to one another. This metamorphism has been pro- 

 duced by the Duluth gabbro, which at a number of places has been found 

 in direct contact with these rocks. One of the best places at which to study 

 the relationship is on the small island crossed by the town line on the 

 southeast side of Gobbemichigamma Lake, and also on the point north of the 

 portage from this lake east into Peter Lake. At these places the sediments 

 are overlain by the gabbro, and the contact line between them can be traced 

 very clearly. The vertical thickness of the contact rock as measured on 

 Gobbemichigamma Lake seems not to have exceeded 50 feet. At many 

 places along the shore there is a horizontal exposure of much more than 

 this in width. This represents, however, the beveled edge of the contact 

 zone, and since no data for the reconstr,uction of the removed material 

 showing the inclination of the surface overlain by the gabbro can be 

 obtained, no measurement can be made of the true width of the contact 

 zone. 



On the island referred to above, and immediately next to the gabbro, 



aGeol. and Nat. Hist. Survey of JMinnesota, Fifteenth Ann. Eept., 1886, pp. 183 and 351. 

 6 Ibid., Seventeenth Ann.. Rept., 1888, p. 130. 

 clbid., Final Kept., Vol. IV, 1899, p. 478. 



