AECHEAN GRANITES. 247 



the general period of intrusion of all of these acid igneous rocks may be 

 placed between the period of the deposition of the latest sediments of the 

 Archean and that of the deposition of the earhest sediments of the Lower 

 Huronian series. Some were intruded near the beginning of this interval, 

 others probably near the end, but it is now impossible to give their exact 

 ages. In the description of the rock from each of the large areas after 

 which it is named an attempt will be made, where there are any facts which 

 warrant this, to determine more closely its period of intrusion relative to 

 the other igneous rocks as well as to the sediments, 



< GEAlSriTE OF VERMILIOK LAKE. 



DISTRIBUTION, EXPOSURES, AND TOPOGRAPHY. 



Distribution. — Granites and related acid rocks are found in great 

 quantity on the islands in Vermilion Lake and on the adjacent shores. 

 They are not, however, confined in their distribution to the immediate 

 vicinity of the lake, for scattered dikes of similar rocks are found cutting 

 through the various older formations in places many miles distant from 

 Vermilion Lake. Direct surface connection of these distant dikes with 

 the main masses can not of course be shown, but from their similarity to 

 the- larger masses in the-area it is presumed that both are derived from the 

 same deep-seated source. 



JExposiires. — The exposures of these acid rocks are very numerous, and 

 many of them afford opportunity for a study of their different kinds, but 

 they rarely show more than a single contact when the contact is between 

 members of the same eruptive series, so that in most cases it is impossible 

 to tell the exact relations of these rocks to one anotlier — that is, to determine 

 which is of younger age. Some of the best places at which to see these 

 Vermilion Lake intrusives are the east end of El}^ Island, the point south 

 of Mud Creek Bay, Stuntz Island, the conical island east of Stuntz, the 

 prominent point of land farther east of this conical island, and the high, 

 bare hills on the north side of the "Burnt Forties." Dikes of these rocks 

 are also numerous in the Ely greenstone north of Mud Creek Bay, where 

 one of them about 30 feet wide, cutting the greenstone and trending east 

 and west, can readily be seen from the water's edge as a white streak along 

 the hillside. 



On the flanks of knolls occupied by the igneous rocks we very commonly 



