136 GEOLOGY OF THE HIGH PLATEAUS. 



lava with extreme rapidity. In the basaltic rocks we have thus, as I 

 believe, most satisfactory evidence that when they reach the surface they 

 are heated to a temperature much above that of mere fusion. In no other 

 way are we able to account so satisfactorily for the persistency with which 

 they retain their extreme liquidity and flow to such great distances. The 

 same fact appears in the study of the minuter textural characters of the 

 basalts. Under the microscope everything indicates an intense degree of 

 ignition. The presence of glass particles and the absence of water cavities, 

 the isotrope base, the exceeding compactness of the rock, its vitreous 

 character, and (in the massive portions) the absence of all traces of 

 viscosity or ropy condition, point to the same conclusion. All this is in 

 strong contrast with rocks of the sub-acid group. The tracliytes and pro- 

 pylites appear to have been erupted, in many cases, in a viscous condition, 

 or in one which was not by any means thoroughly liquid. They are found 

 in thick, cumbersome masses, and, unless the outpour was of excessive vol- 

 ume and mass, do not appear to liave flowed far from their orifices. The 

 trachytes, however, vary much in this respect ; some appear to have been 

 quite liquid, others exceedingly tough and pasty, with all intermediate con- 

 sistencies, though in the most fluent ones there is no evidence of excess of 

 temperature above the point of complete fusion. As a general rule their 

 sluggish character is well pronounced. In the rhyolites there is evidence 

 of intense ignition and thorough fusion ; but the banded, roi^y, and fibro- 

 litic character is suggestive of a temperature just sufficient to melt them to 

 a vitreous consistency, but without that perfect limpid liquidity of the 

 basalts in which the rhyolitic texture would certainly be completely oblit- 

 erated. 



Now, the pyroxenic divisions — the basalts, dolerites, augitic andesites — 

 all betray evidence of superfusion, or a temperature much in excess of 

 that required to melt them. In the hornblendic andesites the same appear- 

 ances are seen, though less in degree. In the propylites they have van- 

 ished, and are not discernible in the trachytes and rhyolites. This is in 

 accordance with the assumption contained in the theory. All rocks more 

 basic than propylite betray evidence of superfusion, and hence it is at 

 propylite in the ascending scale of acidity that superfusion is presumed 



