OCR Text |
Show 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 trachytes 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 volume and mass, do not appear to have 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 consistencies, 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, ropy, 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 obliterated. Now, the pyroxenic divisionsâ€"the basalts, dolerites, augitic andesitesâ€" all betray evidence of superfusion, or a temperature much in excess of that required to melt them. In the hornblendic andesites the same appearances are seen, though less in degree. In the propylites they have vanished, 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 |