602 TRANSACTIONS OP SECTION C. 



the Keewatin sediments were laid down, for they include carbon probably derived 

 from fucoids, which could not have lived in a hot sea. 



The pre-Keewatin land surfaces and sea bottoms have totally disappeared, 

 so far as "known to Canadian geology. Apparently they have been fused and trans- 

 formed into the gneisses of the Laurentian. 



The Keewatin was a time of great volcanic activity, lava streams and ash 

 rocks surpassing in amount the thick sheets of sediments. At the end of the 

 Keewatin the thousands of feet of volcanic and clastic rocks were lifted as domes 

 by the up-welling of batholiths of early Laurentian gneiss. 



Then followed a profound gap in the record, during which the mountains were 

 levelled to a hummocky peneplain. This gap represents a very long period of 

 weathering and destruction on a land surface, ending in glacial action on a large 

 scale. 



The Lower Huronian begins with the deposit of a thick and widespread 

 boulder clay, followed up by a transgression of the sea in which mud and sand, 

 and also limestone and chert, were deposited. 



After a short break similar processes went on in the Middle Huronian. During 

 the Middle Huronian, or in the interval between it and the Upper Huronian 

 (Animikie), mountain-building was renewed on a grand scale, many synclines of 

 Keewatin and Lower Huronian rocks being caught between the rising batholiths 

 of late Laurentian gneiss. A broad central band of the Lower Huronian escaped 

 this process, however, and has preserved its original attitude on a floor of 

 Keewatin and Laurentian. 



The Animikie or Upper Huronian sediments which rest on the planed-down 

 floor of upturned Lower Huronian, Laurentian, and Keewatin rocks consist 

 largely of chert and carbonaceous slate or shale, which lie nearly horizontal and 

 have undergone very little change. 



The Keweenawan follows the Animikie with only a small break, and includes 

 shallow water-beds of sandstone and conglomerate, accompanied by immense out- 

 flows of lava. As a result of the outpouring of lava great basins, like that of 

 Superior, resulted. It is probable that during the Animikie and Keweenawan 

 most or all of the Canadian Shield was covered by the sea. 



The Keweenawan is generally held to mark the close of the Archsean (or 

 Algonkian or Proterozoic). Low reports portions of these formations as having 

 been caught in mountain-building of the Laurentian type in Labrador, but 

 commonly they have not been disturbed. 



During early Palaeozoic times the Canadian Shield was more than once en- 

 croached upon by the sea, though probably much of the peninsula of Labrador, 

 and perhaps a region north-west of Hudson Bay, escaped. 



From the Devonian to the Pleistocene the shield seems to have remained dry 

 land, and part of the Ordovician and Silurian capping of sediments was removed 

 during this long period. 



The succession of Pleistocene ice-sheets completed the work of denudation, and 

 at the end of the Ice Age many thousands of square miles of the lower portions 

 were once more beneath the sea. 



Last of all, the region has been rising at unequal rates in different parts, as 

 shown by the warping of marine and fresh-water beaches. 



The surface of low hills and rounded knolls of gneiss and schists beneath the 

 Pleistocene boulder clay resembles in every way that beneath the flat shales and 

 limestones of the early Palaeozoic, or the nearly horizontal sediments of the 

 Animikie, or even the undisturbed parts of the Lower Huronian boulder clay. 

 It may be that much of the surface has been covered with sediments and restored 

 to daylight by subaerial erosion several times in succession. The greater part 

 of the carving-down seems to have been done before the Animikie — i.e., within 

 pre-Cambrian times — and the pre-Huronian surface seems as mature as any of 

 the later ones. The bearing of this on the length of early geological time is 

 evident. Pre-Huronian time includes the laying down of thousands of feet of 

 Keewatin sediments, the elevation of early Laurentian mountains, and the level- 

 ling of these mountains to a peneplain. It may be as long as post-Huronian time. 



