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University of California Publications in Geology [Vol. 10 



Summary. — It thus appears that in every one of the fifteen dis- 

 tricts, extending over a belt of the continent nearly a thousand miles 

 long and covering both sides of the Great Lakes, we have at least one 

 and not more than two periods of granitic invasion well established. 

 The older granite is represented in all fifteen districts ; while the 

 later granite is found in thirteen districts, failing to appear in the 

 Crystal Falls and Marquette, although in the last-named district more 

 discriminating study may yet show its presence. In neither of the 

 two districts in which only one granite is found is there any doubt as 

 to the period to which it belongs. We may safely conclude, there- 

 fore, that in every one of the fifteen districts, considered individually, 

 geological time is blocked out into three grand divisions by the two 

 granite invasions : the pre-granitic, the inter-granitic and the post- 

 granitic. It is the essence of the hypothesis here set forth that these 

 three divisions of time, which are so well established in the individual 

 districts, are the same in all of them, there being but one pre-granitic, 

 one inter-granitic and one post-granitic period represented by the 

 known epigene rocks throughout the entire region from Rainy Lake 

 to the Adirondacks. 



EVENTS CONNECTED WITH GRANITIC INVASION 



Wherever the earth's crust is known to have been extensively in- 

 vaded by granite, an important concomitant condition has been the 

 uplift of the region affected and the inauguration of a prolonged 

 period of degradation, culminating in the removal of the cover from 

 extensive areas of the granite. Wherever in the course of time sedi- 

 mentation resumed its sway, the resumption was not effective until 

 the region had been reduced to a surface of low relief. The time nec- 

 essary for the invasion of a region by granite is unknown, but it may 

 well have been a long drawn-out process. The stripping of the cover 

 of the granite, however, and particularly the reduction of a high 

 region to low relief, requires a long time in the geological sense ; and 

 the interval of no deposition, between the sediments resting on the 

 worn surface of the granite and the sediments into which the granite 

 is intrusive, constitutes an unconformity of a major order. We may 

 for practical purposes take the appearance of a worn surface of granite 

 upon which as a basement sedimentary strata rest as prima facie evi- 

 dence of a major unconformity. Now we have such a major uncon- 

 formity well revealed on the northwest side of Lake Superior in the 



