I 



GENEKAL GEOLOGY OF THE DISTRICT. 467 



of which has been for the most part determined. The rocks have simply 

 been tihed to the northward at an angle most convenient to determine the 

 succession of belts. It is without any subordinate fold whatever, so that 

 traversed anywhert, excluding the Eastern area and Cherty limestone, if 

 sufficient exposures are found, the succession of its belts can be made out, 

 one following the other in conformity. This series is terminated on the 

 east by the unconformably overlying horizontal Eastern sandstone; it is 

 terminated on the west by its being entirely swept away by erosion, the 

 Keweenaw series directl}' underlying the Southern Complex. It is marked 

 off from the underlying granitic and gneissic rocks by one of the greatest 

 unconformities of geology. The proof of this unconformity, as evidenced 

 by broad relations and by numerous localities in which actual contacts are 

 found, is of the clearest possible sort. To the north of the Penokee rocks 

 is a third independent set of formations, the Keweenaw series, which is 

 separated from the former by a time gap only inferior to that just men- 

 tioned. 



Depth and metamorphisin. — The Penokee series furnishes an instructive 

 lesson as to the depth to which rocks can be buried- and still be slightly 

 affected by metamoi-phosing processes. The series itself is 14,00(J feet thick. 

 It was covered before being upturned by a great thickness of Kewee- 

 naw rocks. This series at the Montreal river is estiniated' to be 50,000 

 feet tliick. Adding this to the known thickness of the Penokee series 

 we have a thickness of 64,000 feet, or more than 12 miles. The Peno- 

 kee rocks were, then, Ijuried to a great depth, the exact amount depending 

 upon their lu)riz()u and upon the stage in Keweenaw time when the tilting 

 and erosion which brought them to the surface was inaugurated. That the 

 s}'nclinal trough of lake Superior began to form before the end of the 

 Keweenaw period, and consequently that the Penokee rocks were not 

 buried under the full succession, is more than probable. However, they 

 must have been buried to a very great depth — at least several miles— and 

 thus subjected to high pressure and temperature, notwithstanding which 

 they are comparatively unaltered. In the quartz-slates near the bottom 



'Copper- Bearing Rocks of Lake Superior; by K. D, Irving, p. 230, U. S. Geol. Survey, Mon- 

 ograpk V. 



