1H80.J on Land and Sea in relation to Geological Time. 277 



pcan side of the Atlantic, tlio mountains wliicli border tlie Oceanic 

 basin corrcsjiond in scale with those on its western border, rather than 

 with those on the Pacific slope of the American continent. On the 

 western side of the Pacific, on the otlier hand, the Malayan Arcliipe- 

 lago constitutes (as already pointed out) a region of extraordinary 

 volcanic activity ; and this is probably the greater on account of the 

 comparative narrowness of this continental plateau, so that it is subject 

 to the lateral thrust of tlie sea-bed of the Indian Ocean in addition. 



But the lateral thrust exerted by these floors, being resisted by the 

 buttresses presented by the continental plateaux, will tend to produce 

 an upward bulging of these floors themselves, especially in their 

 median portion. And this, again, corresponds with fact ; such an up- 

 ward bulging showing itself in the median portion of the bed of the 

 Atlantic, botli north and south ; while the force which raised this, also 

 manifests itself in the volcanic action which has pushed up the Azores 

 and Tristan d'Acunha in corresponding positions. So, in the North 

 Pacific, we have the remarkable volcanic Hawaian group, occupying 

 the same relative position as the Azores in the North Atlantic ; while 

 over the still wider expanse of the South Pacific, there seem to be 

 several of these upward bulgings, that have exploded (so to speak), 

 here and there, in local volcanic action. 



I must not follow Professor Dana's masterly hypothesis into 

 further detail, but must content myself with noticing one point which 

 seems to me of singular interest — namely, the explanation he gives of 

 the depression of portions of what he regards as the original continental 

 platforms, over which long series of sedimentary deposits have been 

 formed, of course implying a subsidence of their base to an amount 

 at least equal to their total thickness. The first step in ordinary 

 mountain-making by lateral thrust, is affirmed by Professor Dana to 

 be a downward bend of the crust, or " geosynclinal." " In the making 

 of the Appalachians, there was first, under the lateral pressure, a 

 slowly progressing subsidence ; it began in, or before, the Primordial 

 period, the commencing era of the Silurian, and continued in progress 

 until the Carboniferous age closed. As the trough deepened, deposits 

 of sediment, and sometimes of limestone, were made, that kept the 

 surface of the region near the water-level ; and when the trough 

 reached its maximum, there were 40,000 feet of thickness of stratified 

 rock in it, and this, therefore, was the depth of the trough. The Green 

 Mountains began in a similar subsidence, and at the same time ; 

 and the trough was kept full with deposits as it progressed. Such 

 facts are in the history of many, if not all mountains." 



The foregoing arguments may be thus combined : — 

 A. The enormous depth of the Oceanic sea-bed, as compared with 

 the height of the Land above the sea-level, renders it very unlikely that 

 any subsidence of a Land-area should be compensated by such an up- 

 lifting of a portion of the Ocean-floor as would raise it above that level. 

 Thus, supposing that all the Land of the globe were to sink down to the 



