Penny Blood: Serial Sensation and Working-Class Entertainment — page 273: what you’re looking at

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

256 SEQUENCE OF VOLCANIC ROCKS. _ [cu. xx. The rocks must be fused. Second, The density of the lavas must be less than that of the overlying rocks. Having shown from independent considerations that the proximate cause of volcanic activity may be a local rise of temperature in deeply seated rocks, it only remains to follow the obvious phases of the process.” He then founds, upon a previous discussion of the respective temperatures at which the different rocks would become at once fusible and sufficiently light to reach the surface, an argument to show that Richthofen’s order of succes- sion agrees with the order so arrived at. “It is just possible that the acid rocks may be light enough to erupt at an early stage of the process””—of a local rise of temperature—“ but are not yet melted, and that the basic rocks may be melted, but must await a further expansion in order to reach the surface. The first selection would then fall upon some intermediate rock.’” Several objections to this explanation are mentioned in a short critique upon this, in general admirable, Work by Prof. A. Geikie’. But the fundamental objection already brought against it, of the impossibility of explaining the “local rise of temperature” pos- tulated, seems to us fatal’. It appears however to Prof. Judd that the exceptions to Richt- hofen’s law as precisely stated, “are so numerous as to entirely destroy its value. The generalisation that in most volcanic dis- tricts the first ejected lavas belong to the intermediate group of the andesites and trachytes, and that subsequently the acid rhyolites and the basic basalts made their appearance, is one that appears to admit of no doubt, and is found to hold good in nearly all volcanic regions of the globe which have been attentively studied”*. But even this generalisation is sufficient to necessitate some explanation. Prof. Judd says also that “‘it lends not a little support to the view that under each volcanic district a reservoir of more or less completely molten rock exists, and that in these reservoirs various changes take place during the long periods of igneous activity. During the earlier period 1 “Geology of the High Plateaus of Utah,” p. 131. 2 **Nature,” xx. p. 824. 1880, 3 p. 241, * ** Volcanoes,” p. 200. commiclsoolkks (CO) in