DRY LAND IN GEOLOGY COLEMAN. 267 



tian peneplain. In some places the tillite passes downward, with 

 no visible break, into an old regolith due to the decay of the Lau- 

 rentian gneiss or Keewatin greenstone beneath. In others the rock 

 below has been smoothed and polished, though no striae have yet been 

 found on it. 



It is impressive to come on this old land surface half way down 

 in the pre-Cambrian succession, yet as thoroughly baseleveled as the 

 neighboring undulating surface of gneiss and greenstone, from which 

 rain and frost are now stripping the bowlder clay. The continent 

 sealed up beneath the Huronian tillite looks as finished and as ancient 

 as the Laurentian peneplain beneath the bowlder clay of the last ice 

 age. The strenuous history of the world since Huronian days could 

 add nothing appreciable to its hoary antiquity. Great mountain 

 ranges had already been gnawed down to the bare crystalline founda- 

 tions before the ice of the Huronian covered the surface with bowlder 

 clay, and this all happened long before a trilobite was entombed in th(j 

 mud of a Cambrian sea. 



Though the extent of the Huronian ice sheet is only imperfectly 

 known, it is certain that a plain in all respects like that beneath the 

 tillite stretches 2,000 miles northwestward to the Arctic Ocean and 

 more than 1,000 miles northeastward to the edge of Labrador, for 

 flat-lying areas of Animikie or Keweenawan rocks cover a dozen 

 broad areas of similar peneplain in other parts of the Canadian 

 Shield. The same plain slips gently imder Silurian and Devonian 

 sediments in the central depression of Hudson Bay, under Ordo- 

 vician limestone and Potsdam sandstone in Ontario, and under 

 Silurian, Devonian, and Cretaceous rocks toward the southwest. 

 How far the unchanged pre-Huronian peneplain or its little changed 

 successor extends southwestward beneath the stratified rocks is un- 

 known. 



Much of this vast surface has been buried at one time or another 

 and sheltered from erosion by marine sediments, and has since been 

 disinterred scarcely modified, but it is probable that it was never 

 all covered by the sea at once. Portions of it seem to have re- 

 mained dry land as cities of refuge for the inhabitants in every 

 inundation. 



That other continental nuclei have had similar histories may be 

 considered certain. In Scotland and Scandinavia nearly horizontal 

 pre-Cambrian beds, whether of glacial origin or not, cover a pene- 

 plain closely like ours, and quartzites and conglomerates called pre- 

 Cambrian may be seen resting with gentle dips on a similarly trun- 

 cated plain in West Australia. Near Clackline, for instance, 

 Huronian-looking quartzite rests on gneiss penetrated by pegmatitt; 

 dikes, and at several places in the neighborhood of Kalgouiiie and 



