6() THE POST-PLIOCENE PERIOD. 



The facts to be accounted for are the striation and polishing of 

 rock surfaces, the deposit of a sheet of unstratified clay and stones, 

 the transport of boulders from distant sites lying to the northward, 

 and the deposit on the boulder clay of beds of stratified clay and sand, 

 containing marine shells. The rival theories in discussion are, — -first, 

 that which supposes a gradual subsidence and re-elevation, with the 

 action of the sea and its currents, bearing ice at certain seasons of the 

 year ; and, secondly, that which supposes the American land to have 

 been covered with a sheet of glacier several thousands of feet thick. 



The last of these theories, without attempting to undervalue its 

 application to such regions as those of the Alps or of Spitzbergen or 

 Greenland, has appeared to me inapplicable to the drift deposits of 

 eastern America, for the following among other reasons : — 



1. It requires a series of suppositions unlikely in themselves and 

 not warranted by facts. The most important of these is. the coin- 

 cidence of a wide-spread continent and a universal covering of ice 

 in a temperate latitude. In the existing state of the world, it is well 

 known that the ordinary conditions required by glaciers in temperate 

 latitudes are elevated chains and peaks extending above the snow- 

 line ; and that cases in which, in such latitudes, glaciers extend nearly 

 to the sea-level, occur only where the mean temperature is reduced 

 by cold ocean-currents approaching to high land, as for instance in 

 Tierra del Fuego and the southern extremity of South America. But 

 the temperate regions of North America could not be covered with 

 a permanent mantle of ice under the existing conditions of solar 

 radiation ; for, even if the whole were elevated into a table-land, its 

 breadth would secure a sufficient summer heat to melt away the ice, 

 except from high mountain-peaks. Either, then, there must have been 

 immense mountain-chains which have disappeared, or there must have 

 been some unexampled astronomical cause of refrigeration, as, for ex- 

 ample, the earth passing into a colder portion of space, or the amount 

 of solar heat being diminished. But the former supposition has no 

 warrant from geology, and astronomy affords no evidence for the latter 

 view, which, besides, would imply a diminution of evaporation mili- 

 tating as much against the glacier theory as would an excess of heat. 

 An attempt has recently been made by Professor Frankland to account 

 for such a state of things by the supposition of a higher temperature 

 of the sea, along with a colder temperature of the land; but this 

 inversion of the usual state of things is unwarranted by the doctrine 

 of the secular cooling of the earth ; it is contradicted by the fossils of 

 the period, which show that the seas were colder than at present; 

 and if it existed, it could not produce the effects required, unless a 



