50 Mr. E. W. Hilgard on some points in 



but may be obviated by admissible assumptions regarding the 

 mode of distribution of the solid and liquid matter constituting 

 the globe, — we are led to the reasonable assumption that while 

 the thickness and rigidity of the crust is evidently too great to 

 admit of further folding or fissure-eruptions, and (probably) to 

 admit of connecting ordinary volcanic phenomena directly with 

 the (virtually or actually) liquid interior, yet we need not as- 

 sume it to be so great as to render the crust incapable of yield- 

 ing somewhat, on a large scale, to static upward pressure. Such 

 pressure may be either the resultant of tangential stress, such 

 as might slightly deform an arch without fracture or folding, 

 or even the direct result of a corresponding subsidence else- 

 where. 



The latter effect would of course be incompatible with a 

 shrinking away of the fluid interior from the crust, as required 

 by Mallet's theory, if it were necessary to assume that the in- 

 terior crust-surface is substantially " smooth/'' i. e. free from 

 important downward projections or upward sinuosities- But so 

 far from this, the cooling influence that has so long acted on 

 the oceanic areas, contrasted with those enormous outwellings 

 of igneous rock that have occurred even in late Tertiary or 

 Posttertiary times, together with other considerations, necessi- 

 tate the assumption that such inequalities do exist to a notable 

 extent. Hence the overlapping alluded to by Mallet of the 

 period of fissure-eruptions and of that of volcanic activity 

 proper, which appear to have coexisted, in different portions of 

 the globe, from early Tertiary to early Quaternary times. For 

 even Mallet himself considers the outpourings of igneous rocks 

 on the Pacific coast "wholly inconsistent with existing vol- 

 canic forces ;" and few geologists will agree with LeConte * in 

 ascribing precisely these most extensive fissure-eruptions in the 

 world to the " ineffectual fires " of the volcanic period, arising 

 alone from transformed motion. 



Indeed it is not easy to understand the precise mechanism 

 of the great fissure-eruptions as a consequence of nucleal con- 

 traction, without the aid of some static head of pressure that may 

 exist more or less locally, in consequence of inequalities in the 

 crust (whether of form, thickness, or density), and thus act as 

 a vis a tergo. 



At first blush the " squeezing out of sub-mountain liquid 

 matter," assumed by LeConte as the consequence of the fold- 

 ing and Assuring of strata by tangential thrust, appears natural 

 enough. Yet it seems hardly possible that the same force 

 which makes and elevates mountain folds (being the result of 

 interior shrinkage) should at the same time serve to cojnpress the 

 * Silliman's American Journal, Maich 1874, p. )"9. 



