PRESIDENTIAL ADDRESS. 597 



upturned edges of the synclinally disposed Keewatin schists and the truncated 

 domes of Laurentian gneiss. 



The Huronian. 



The Lower Huronian has very different relationships from the Keewatin. 

 Where least disturbed, as north of Lake Huron and in the Cobalt region, the 

 floor beneath it is often well preserved. Dr. Miller has shown that at Cobalt the 

 surface of Keewatin and Laurentian was hilly or hummocky before the basal 

 conglomerate of the Lower Huronian was deposited ; and Professor Brock, in 

 describing the Larder Lake district to the north, refers to ' the clean-swept and 

 often rounded surface of the older rocks on which it is frequently laid down.' 1 



The basal conglomerate of the Lower Huronian contains pebbles and boulders 

 of all the Keewatin and Laurentian rocks that went before, and among them 

 are found beautifully striated stones. It is the oldest known boulder clay or 

 tillite. The vast period of subaerial destruction that carved away the early 

 Laurentian mountains ended in a glacial period, whose ice-sheets covered many 

 thousands of square miles of North America, just as the last great period of 

 peneplanation ended with the Pleistocene ice-sheets. 



It is not a little impressive to see modern till resting on the Huronian tillite 

 and including fragments of it as boulders. It is possible to break out from the 

 modern glaciated surface stones whose underside received their polish and striae 

 in the Lower Huronian, while their upper surface has been smoothed and 

 scratched by Pleistocene ice movements. 



At Cobalt the tillite is accompanied by slate, which may be compared in all 

 essential characters except hardness with the stratified clay of adjoining lake 

 deposits of Pleistocene Age. The most recent and unconsolidated beds make 

 clear the origin of some of the most ancient and, in appearance, most different 

 rocks in the world. 



In the Lake Huron region the action of ice was probably followed by an 

 invasion of the sea, for the tillite is succeeded by thousands of feet of quartzite, 

 arkose, and conglomerate, and by a few hundred feet of limestone. Possibly much 

 or all of the limestones of the Grenville and Hastings series, which Dr. Adams 

 reckons among the great limestone formations of the world, were formed at about 

 the same time. 



The Middle Huronian (Logan's Upper Huronian) is separated by a basal con- 

 glomerate, possibly glacial, from the Lower Huronian; but the break does not 

 seem very profound, and the rocks do not differ much from those just described. 



The least changed parts of the Huronian extend as a wide band for 200 or 

 300 miles north-east of Lake Huron, and in this area the uneven surface of 

 Laurentian and Keewatin beneath the Lower Huronian boulder clay preserves for 

 us a portion of the earliest dry land, the earliest peneplain known in America, and 

 possibly in the world. This band has remained comparatively stable, while, so 

 far as our information goes, all other parts of the Canadian Shield have undergone 

 violent changes. 



Rise of the Late Laurentian Mountains. 



The Lower Huronian tillite has been found in many places throughout the 

 Archaean region, over a stretch of 1,000 miles from east to west, and 700 miles 

 from north to south ; so that in all probability deposits like the Pleistocene till 

 covered most of the surface. 



Everywhere, however, except in the band extending north-east from Lake 

 Huron, it seems to have been involved in later mountain building, and has been 

 so sharply folded in with the Keewatin as to destroy the appearance of uncon- 

 formity. It is instructive to note that so long and momentous an interval was 

 entirely overlooked by geologists or treated as of small importance until a few 

 years ago. There is usually no angular discordance to be observed, and the 

 secondary schistose structures of Keewatin and Huronian are similar and parallel. 

 The Huronian boulder conglomerate has often been rolled out to a schist in which 

 only the harder boulders can be recognised as lenses; and sometimes even they 

 are lost entirely, so that no evidence of discordance remains. 



1 Bur. Mines, Ont., 1905, p. 31. 



