THE OLD AGE OF CONTINENTS. 123 



fulfill their mission in the economy of matter and of life; 

 the furrows of senescence channel their wasted faces, and 

 they return to mud and slime whence they were born. 

 The very substance of the solid floors which underlie the 

 soil of American freedom, is but the dust of continents 

 decayed. As modern cities are sometimes built from the 

 ruins of ancient temples on whose sites they stand, so 

 the dwelling place prepared for man by the hand of Na- 

 ture is but the reconstructed material of a more ancient 

 continent, the work of Nature's " ' prentice hand." The 

 vertical thickness of fifty thousand feet of sedimentary 

 strata measures the depth of the rubbish accumulated 

 from mountain cliffs and continental slopes that have 

 been transformed by the wand of time. We sometimes 

 forget that the total volume of our stratified rocks is 

 but an index of the denudations and obliterations that 

 have been wrought. Much calcareous material has, in- 

 deed, been yielded by the sea, but the sea first filched 

 it from the land. 



The revelation made by every formation which we 

 study, from the bottom to the top of the Palaeozoic series, 

 points to the north and northeast as the origin of the 

 stream of sediments that spread over the bottom of the 

 American lagoon which stretched as a broad and shallow 

 ocean from the rising but yet submarine slopes of the Alle- 

 ghanies on the east, to the embryonic ridges of the Rocky 

 Mountains on the west. Northeastward of the present 

 continent have undoubtedly existed supplies of incalcula- 

 ble magnitude of which the merest vestiges remain. The 

 geologist leads us to the region north of the great lakes 

 and the St. Lawrence river, and points out the Lauren- 

 tide Ridge as the nucleus of the eastern portion of our 



