On the Agency of Water in Volcanic Eruptions. 161 



Where liave we evidence, even in the most recent of mountain chains, 

 when the earth was approaching its present conditions of rigidity, of 

 a shell of 700, or 500, or 100, or even of 50 miles thick ? Would it 

 not rather appear that a crust of 30 miles is even in excess of what 

 the height and breadth of any mountain chain would, on this finding, 

 indicate ? 



To the first part of the argument, it may be rejoined that the 

 existence of a thinner crust and of a fluid nucleus is not contested in 

 the various geological periods, and that the conditions of solidity and 

 rigidity are only applicable to the globe as it at present exists. But 

 the observation loses its point when we consider that the cooling must 

 have been slow and gradual throughout all time, — that the formation 

 of mountain chains has been intermittent with long intervals, — that 

 the last-formed chains show no change in the character or diminution 

 in the forces to which they are due, — and that there is nothing to 

 indicate such a sudden accession of solidity in the earth as would be 

 involved by the assumption of the free play of the crust during 

 Tertiary times and its entire rigidity now ; whilst, on the other hand, 

 other forms of the forces coordinate in a cooling globe still continue 

 in visible action. If one form of a force dependent on a common 

 cause remains in operation, we are scarcely justified in assuming that 

 another consequence of the same cause, though dormant, is extinct. 

 Our limited experience suffices to make us acquainted with the 

 more persistent effects, but fails to compass those which are inter- 

 mittent. 



b. With respect to the other analogous forces in operation now and 

 in times immediately preceding our own, they likewise indicate a 

 yielding substratum — although the disturbances at present caused by 

 them may not result in the fractures and contortions involved in 

 mountain-forming, and which are only the effects of prolonged and 

 extreme tension. I allude to those wide-spread movements which 

 result in great superficial or continental upheavals— movements 

 which, of frequent occurrence in all geological times, have not 

 altogether ceased in our own times. Our object is, however, with the 

 later periods only. It will be sufficient if we go back towards the 

 close of the Tertiary period. 



We have merely to look at a geological map of the world to see 

 that a very large portion of the existing continents have been under 

 the sea during the Tertiary period. South-eastern England, a large 

 portion of France, great part of Spain and Italy, and the whole of 

 Central Europe, have, since the Eocene period, undergone movements 

 of elevation en masse with little disturbance to the strata over large 

 areas, and these have been prolonged down to Miocene and Pliocene 

 times. In the same way the elevation above the sea of great part of 

 Asia Minor and Mesopotamia is of Post-Miocene age. Nor has the 



VOL. XLI. M 



