52 On some points in Mallet's Theory of Vulcanicity. 



case, we assume the existence of a thickly viscid, igneoplastic 

 undercrust layer. Such a layer, while barely or very slowly 

 obeying the laws of liquid equilibrium, would be capable of 

 being liquefied by a slight increase of temperature, such as 

 might be produced by squeezing or kneading. Portions of 

 such plastic matter would occasionally become iuvolved in the 

 anticlinal folds of synclinoria, and thus supply the material for 

 limited fissure-eruptions, in that case literally " squeezed out." 

 But the inverse ratio pointed out by Dana as existing between 

 folding and fissure-eruptions points to the rarity of such 

 events. 



At any rate they could not explain the outwellings of the 

 Pacific border, which continued long after close plications had 

 ceased to be made — in fact, as it would seem, up to the end of 

 the period of elevation of the main Sierra Nevada. 



It is but fair to assume that near lines of weakness indicated 

 by plications or fissure-eruptions, the isogeotherms have been 

 during the elevation of mountain- chains (and probably still are 

 where such lines are marked by volcanic vents) considerably 

 above their general level. In an anticlinal upheaval they 

 would probably conform to the progress of the sublevatory 

 movement, in a ratio more or less directly proportional to the 

 rapidity of the upward movement, and would gradually descend 

 during periods of repose. This would happen independently of 

 any heat generated by transformation of motion. 



In a polygenetic chain like the Sierra Nevada, after the col- 

 lapse and folding of the geosynclinal and the subsequent stif- 

 fening of the backbone (so to speak), any further elevation of 

 the main ridge becomes a quasi-anticlinal movement, accom- 

 panied necessarily by the compression and " squeezing " of the 

 heated rocks embraced within the arch. The heating being 

 greatest, cater is paribus, where the resistance and motion is a 

 maximum, more heat would be generated by the compression 

 of the upper, half-stiffened portion of the viscous or igneoplastic 

 layer, than in the lower ones ; and the liquid matter so formed 

 would constitute a head of pressure, from which fissure-erup- 

 tions might derive their material; whether directly, or by pres- 

 sure communicated to more distant points of rupture and fusion 

 by lateral stress. 



If, then, as LeConte's data seem to show, the final and most 

 considerable anticlinal elevation of the great interior range took 

 place during the same period that witnessed the great fissure- 

 eruptions of the Coast and Cascade ranges, it may not be un- 

 reasonable to suppose these events to have not only been con- 

 temporaneous, but to have borne to each other something of 

 the relation of cause and effect, and that each of the numerous 



