THE TOOTH OF TIME. 339 



but about twelve hundred feet above the level of the sea; 

 and if the entire continent were ground to powder down to 

 the sea-level, and distributed over an area of the ocean's 

 bottom equal only in extent to North America, it would 

 afford a bed of strata not one twentieth the thickness of 

 the Laurentian system over the same region. Whence, 

 then, the materials for so vast an accumulation of sedi- 

 ments? Where were the lands which must have disap- 

 peared during the Laurentian Age? Although we may 

 not be able to indicate their location, the facts suggested 

 serve to remind us of the gigantic scale of operations of 

 the denuding agencies of primeval time. 



Every succeeding geological age must also have had its 

 source of supply to the contemporaneous sediments. The 

 ever-growing continents were ever wearing down. As the 

 increasing pressure of the accumulating oceans crowded 

 higher the summits of the continental axes, the . ceaseless 

 demands of the insatiate sea for more sediments wore 

 thinner and thinner their denuded scalps. It is no wonder 

 that included fires burst forth at the summits of the high- 

 est mountains. These are the exposed points, where the 

 earth's crust has been reduced to the greatest degree of 

 tenuity, while the ocean's floor is the most solid portion 

 of the globe. 



The outburst along the southern shore of Lake Superior 

 at the close of the Potsdam period developed topograph- 

 ical features of infinitely greater ruggedness than those 

 which now characterize that region. Kewenaw Point, the 

 Porcupine Mountains, and the Huron Mountains, as well as 

 the numberless unnamed knobs still standing throughout 

 the region, have been gnawed and battered down for hun- 

 dreds of feet, and their once angular outlines have been 

 scoured to a subdued rotundity. The Appalachians, that 

 once lifted their multiplied folds to the heights of the An- 



