THE PAST AND FUTURE OF GEOLOGY. 181 



due to gravitation and contraction, from tbe action of subterranean 

 forces, from tbe effects of beat, and witb all tbe varied pbeuomeua re- 

 sulting from tbese com[)lex conditions. 



Passing for tbe present over tbe intermediate stages of consolidation, 

 tbe continual cooling of tbe globe bas necessarily resulted in atbicken- 

 iug of its crust, tbe exact extent of wbicb at tbe present time bas long 

 been tbe object of geologists to determine. 



Tbe inquiry is one of extreme difiticulty, and bas of late years engaged 

 and is stiil engaging tbe attention of some of tbe ablest pbysici.sts and 

 matbematicians. Tbe early belief was tbat tbe tbickness of tbe crust ot 

 tbe eartb does not now exceed tbirty to sixty miles; but tbe late Mr. 

 Hopkins, reasoning on pbenomeua connected witb precession or nuta- 

 tion, concluded tbat on tbe contrary it could not be less tban eigbt liun- 

 dred miles tbick, or more; a conclusion wbicb bas been supported and 

 extended by Sir W. Tbompson, wbo, wbile maintaining tbe igneous ori- 

 gin of tbe globe, and tbe greater intensity of action in past ages, bas 

 furtber proved, on dynamical grounds, tbat tbe eartb as a wbole must 

 now be more rigid tban glass, and probably even more rigid tban steel. 



It is difficult, bowever, to reconcile tbese views witb tbe extent and 

 character of recent volcanic action. Tbis Mr. Robert Mallet endeavors 

 to do, in a remarkable paper recently publisbed in tbe Transactions of 

 tbe Royal Society. Tbe autbor bases bis views upon Constant Prevost's 

 tbeory of elevatory forces, but considers tbat as tbe secular cooling of 

 tbe globe bas proceeded, and tbe crust become tbicker and more rigid, 

 tbe tangential pressure, no longer equal to tbe elevation of mountain- 

 ranges, is spent in local crusbings of portions of tbe crust; and tbat, by 

 transformation of tbe mechanical work of compression, tbe beat iroui 

 wbicb terrestrial volcanic agency is at present derived is protluccd. 

 Mr. Mallet contends for tbe bigh probability tbat tbis " crushing of the 

 earth's solid crust affords a supi)ly of energy sufficient to account for 

 terrestrial vulcanicity," comprehending in tbat term eartbipiakes and 

 volcanic action. Thus, instead of arising from a deep-seated and com- 

 mon cause, Mr. Mallet would assign present volcanic ejections to tbe 

 local fusion of tbe strata at variable but moderate depths beneath the 

 surface ; and be considers it characteristic of such action " tbat it is 

 only one phase of a unique force wbicb bas always been in action, 

 though in decreasing energy, since our planet was nebulous." 



On the other band, these views have been objeiited to by other com- 

 petent observ^ers, wbo hold witb little nioditication to tbe original 

 hypothesis of a molten central nucleus and a shell of comparatively small 

 thickness. Such are some of tbe lai-ge ])hysical problems now oeeupy- 

 iug tbe attention of geologists. I shall have occasion to recur to them 

 again. 



In stratigrapbical geology, tbe great (I*ivisions originally introduced 

 by our predecessors stand, but their number and the inunber of sub- 

 divisions have greatly increased. In 1822, when Phillips and Conybeare 

 wrote their "Geology of England and Wales," twenty-three so-called lor- 



