Penny Blood: Serial Sensation and Working-Class Entertainment — page 271: what you’re looking at

Machine-transcribed from the original scan — historical spelling and the odd misread are preserved.

254 SEQUENCE OF VOLCANIC ROCKS. [cH. Xx. couches, superimposed in reverse order of their specific gravities. The manner in which Richthofen explains this succession is satisfactory as regards the first and last, but not equally so as regards the intermediate rocks. Premising that the eruptive rocks of palzozoic times were chiefly of an acid character, and that little eruptive activity was manifested during the mesozoic, he proceeds’, “Those siliceous compounds especially, of low specific gravity, which had formerly yielded the material of the vast accumulations of quartziferous eruptive rocks, would have been consolidated, and the limit as 1t were between the solid and the viscous state of aggregation receded into regions where the matter would be of a less siliceous com- position and of greater specific gravity.” * * * When the eruptive activity was renewed, “The first rocks ejected would neces- sarily be of a more basic composition than the predominant rocks of the granitic era, while the repetition, at a later epoch, of the process of fracturing would give rise to the ejection of rocks in which silica would be contained in still lower proportion. The greater portion indeed of the ejected rocks consisted of propylite and andesite, in the first, and of basalt in the second half of the volcanic era.” Thus far all is plain. But when he offers an explanation of the intermediate ejection of a more acid and lighter rock than any of these his reasoning is less convincing. “A notable but only apparent anomaly in the regular order of succession has been the emission of trachyte and rhyolite, between the andesitic and basaltic epochs. But if it 18 considered that these rocks were ejected partly from the same fractures through which andesite had ascended, and partly from others in their immediate vicinity, while the distribution of basalt has been independent, to a certain extent, of all fore- going eruptions, it is evident that the occurrence of trachyte and rhyolite is closely dependent on that of andesite and bears only a very remote relation to basalt. It appears that after the ejection of the chief bulk of andesite, when other processes ending in the opening of fractures into the basaltic region were being slowly prepared in depth, the seat of eruptive 1 “ Natural system of Volcanic Rocks,” p. 58. COMIC OOOKS.EO