Mallet's Theory of Vulcanicity. 49 



the movements as taking place exclusively within the solid 

 shell, he (unnecessarily as it seems to me) leaves a point open 

 to objection. 



While admitting that slow secular oscillations, or those minor 

 changes of level constantly occurring in volcanic areas, may 

 even now in many cases be reasonably attributed to changes of 

 temperature occurring within the solid rocks themselves, and 

 within their limits of elasticity, it is impossible to assign this 

 as an adequate cause of those extensive oscillations which 

 have characterized the Quaternary period, and are recorded, 

 e - 9-> by the raised beaches of the North-Atlantic coasts and 

 inlets, and by the drift-pebbles even now found four hundred 

 and fifty feet below the level of the Gulf of Mexico, while the 

 emerged formations record a complementary elevation to at 

 least a similar extent during the Terrace epoch. This record 

 of an oscillation of near a thousand feet on the Gulf-shore 

 since the glacial-drift epoch, implies at least a corresponding one 

 over the greater portion of the area drained by the Mississippi, 

 unless that river flowed backward at one time*. Doubtless 

 these oscillations, like the glaciation of which they probably 

 were cooperative causes, were of continental extent, as was the 

 (more or less contemporary) emergence of the Siberian plain ; 

 and as such they must be presumed to have been true move- 

 ments of the earth's crust, although lying quite within the vol- 

 canic period proper. It is but reasonable to suppose that the 

 sinking of the great Pacific area was then, and may still be, 

 of a similar nature. 



If Mallet's theory, as well as the geological facts with which 

 it deals, is incompatible with Hopkins's and Thomson's postu- 

 late of extreme rigidity ; if, as it appears to me, the events of 

 very recent geological epochs in connexion with the very slow 

 rate of cooling since that time render it unlikely that the crust 

 can even now be considered rigid in a geological sense; if, 

 finally, as General Barnard affirms, the astronomical objection to 

 a comparatively pliant crust and liquid nucleus is not absolute, 



* It is a curious fact thab in the various hypotheses regarding the oscil- 

 lations of the continental interior during the Drift epoch, the facts ob- 

 served on the Gulf-shore have over and again been quietly ignored, 

 although the Gulf is unequivocally the natural reference-level most directly 

 related to that interior, nut only at the present time, but, as the direction 

 of the Drift currents and the trend of the formations show, ever since the 

 time of the Cretaceous emergence. Nevertheless the reference- level has 

 been sought beyond the Alleghany upheavals, or beyond the fixed Azoic 

 area upon which the movement appears, in a measure, to have pivoted, 

 and where, as Dana has shown, it was materially diminished in extent. 

 Assuredly no hypothesis which disregards the changes of level registered 

 at the continental outlet has any raison d'etre ! 



Phil. Mag. S. 4. Vol. 48. No. 315. July 1874. E 



