Time-table for North America. 23 



They are distinctive in their wide-spread extent and seem to have 

 developed on a scale and with an intensity that are characteristic 

 of those early ages, but are not unique in time. 



This point of view suggests the possibility of a still more 

 distant vista of geologic history. The Coutchiching of Lawson, 

 long doubted but determined recently to have in part a real 

 existence, lies conformably beneath the Keewatin lavas. The 

 Grenville, perhaps the thickest of known sedimentary series, 

 is torn and injected by granite and is generally agreed to have 

 preceded the Paleolaurentian revolution. There is no certainty 

 as yet, however, regarding its relation to the Keewatin. Miller 

 and Knight in the Madoc area have found a series of rocks 

 resembling the Grenville and overlying a series of greenstone 

 schists correlated with the Keewatin greenstones. As the 

 Canadian investigators have pointed out, however, the Grenville 

 is so generally separated from the Keewatin of the Lake 

 Superior succession by a belt of batholiths that no convincing 

 general correlation between the two regions is yet possible. In 

 either case, whether older or younger than the Keewatin, the 

 vast thickness and sedimentary character of the Grenville series 

 would seem to mark out those rocks as the records of a period 

 or era which should ultimately be separated from the Keewatin. 

 The Grenville shows how very far removed in time is the 

 following Laurentian crustal disruption from the origin of the 

 earth. Before that recurrence of the reign of lire, the orderly 

 processes of air and water had been in operation for ages, 

 making and depositing sediments whose thickness is measured 

 by many tens of thousands of feet. The limestone in the Gren- 

 ville. estimated at more than nine miles in thickness, and by far 

 the greatest in amount of any known pre-Cambrian formation, 

 testifies further to the efficiency of chemical weathering, a 

 process frequently inefficient in later pre-Cambrian ages. 



The Grenville everywhere floats upon and is torn to pieces 

 by younger igneous rocks. Its original floor may be everywhere 

 destroyed. Deep erosion has now removed all but the more 

 downsunken troughs or included masses. The interior forces 

 generated in the depths of the earth and the external forces 

 born of the ancient sun thus limit, as if to a glimpse between 

 two curtains, our vision of this oldest known period of the 

 earth. But the history which the Grenville records shows 

 that even this is not the primal eon. That is hidden, perhaps 

 forever, from vision and from hammer. 



