CLARENCE KING'S VOLCANIC HYPOTHESIS. 209 



circumstance that these lavas are superimposed on each other in great 

 thickness over broad areas, is some evidence that no denudation 

 adequate to liquefy rock in so grand a scale could have taken place. 



It was suggested by Scrope that separation of crystalline sub- 

 stances in the magmas might take place, so that specific gravity would 

 exercise an influence in arranging igneous materials in order of their 

 densities ; and Clarence King also takes it to be certain that the 

 crystals in lavas were mostly formed before eruption. Hence, each 

 of these supposed fiery lakes is believed to have been a cauldron in 

 which the light quartzose rock floated towards the surface and was 

 erupted first ; and the heavy augitic rock sunk towards the bottom, 

 and was erupted last, so as to account for the threefold type of each 

 rock given in the table. These views seem to rest on the facts ob- 

 served by Charles Darwin in the Galapagos Islands, that crystals 

 were abundant in the lower part of a lava stream and wanting above, 

 and this circumstance has been observed again by King in the lavas 

 of Kilauea. But although the fractured crystals and fluxion structure 

 of rhyolites prove that the crystals formed before the mass become 

 absolutely solid, the Galapagos and Kilauea observations are more 

 likely to show that the lavas cooled more slowly where in contact 

 with the mountain than when exposed to the air ; and, therefore, that 

 the crystals were produced under conditions of slower cooling in har- 

 mony with all other observations. And if we were once to admit the 

 formation of crystals in the heated rock and their arrangement accord- 

 ing to specific gravity as supposed, it would be difficult to see why 

 the process should not have gone a step further, by separating the 

 minerals themselves, and obliterating the magmas or varieties of the 

 volcanic rocks. The sequence manifested by the American rocks of 

 the Fortieth Parallel certainly needs explanation ; but Clarence King's 

 hypothesis, though preferable to Richthofen's, which would derive the 

 lavas from successively lower zones of the earth's interior, leaves much 

 to be desired. 



If we compare the varieties of granite with the varieties of any of 

 Clarence King's volcanic rocks, we might easily construct a parallel 

 series, except that the augitic member would be wanting ; but the 

 absence of such a rock may not be thought unaccountable when we 

 remember that according to Mitscherlich, Berthier, and G. Rose, 

 the actinolite and grammatite varieties of hornblende, when fused 

 in a porcelain furnace, yield crystals which have the form of augite. 1 

 And, therefore, the granites, hornblendic granites and syenites, might 

 be held to present the same gradations from acidic to basic types, 

 which are seen in the American groups of volcanic rocks, The other 

 igneous rocks may be similarly arranged. 



Relation between Plutonic and Volcanic Rocks. If lavas are 

 poured out on the surface, we may fairly conclude that parent masses 

 of the same rocks consolidate beneath the surface under different con- 

 ditions of temperature, pressure, and liquefaction. And if corre- 

 spondence in chemical composition can be established between certain 



1 " Phillip's Mineralogy," by Brooke and Miller p. 302. 

 VOL. I. O 



