HURONIAN ICE AGE 187 



dian Shield, unless the tillite reported by Hintze from the Wasatch Moun- 

 tains and that from Simla in India are to he referred to so early an age. 

 A characteristic tillite with well striated stones has been found in the 

 famous Cobalt region, its hard boulder-clay cut by the richest veins of 

 native silver in the world. Striated stones have been found also 60 miles 

 to the east, in the Province of Quebec, by members of Morley Wilson's 

 geological survey party, 13 and one from the original Huronian region, 

 160 miles to the southwest, has been figured by Collins. 16 Areas of sim- 

 ilar coarse boulder conglomerate or tillite, sometimes inclosing blocks 

 tons in weight and miles from their source, have been mapped at various 

 points as far northeast as Chibougamau, 320 miles from Cobalt, and have 

 been found also to the west of Cobalt. They are widely scattered over 

 the Canadian Shield and were once much more extensive, covering no 

 doubt many thousands of square miles. 



In most places the tillite rests with gentle dips on the low hills and 

 shallow valleys of a peneplain closely resembling the present Laurentian 

 peneplain. In some places the tillite passes downward, with no visible 

 break, into an old regolith due to the decay of the Laurentian gneiss or 

 Keewatin greenstone beneath. In others the rock below has been smoothed 

 and polished, though no striae have jet been found on it. 



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

 Precambrian succession, yet as thoroughly baseleveled as the neighboring 

 undulating surface of gneiss and greenstone from which rain and frost 

 are now stripping the boulder-clay. The continent sealed up beneath the 

 Huronian tillite looks as finished and as ancient as the Laurentian pene- 

 plain beneath the boulder-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 foundations before the ice of the Huronian covered 

 the surface with boulder-clay,- and this all happened long before a trilo- 

 bite was entombed in the 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 under Silurian and Devonian sediments in the central de- 

 pression of Hudson Bay, under Ordovician limestone and Potsdam sand- 



15 G. S. C, Mem. 39, pp. 88-97. 



10 G. S. C, Museum Bull., No. 8, plate i. 



