Penny Blood: Serial Sensation and Working-Class Entertainment — page 305: what you’re looking at

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

288 SOMMARY. [CH. XXIL which are involved in the above suppositions, and we have ' made an attempt to explain the mechanics of an eruption, and the sequence of events connected with it. For the details of this part of the subject the reader is referred to the body of the book, since they cannot be very easily epitomized. XX. The downward protuberances of the crust into the fluid substratum, which we have termed the roots of the moun- tains, will be gradually melted; and the material melted off, being lighter than the substratum, will flow upwards, and spread itself beneath the crust. This fact appears capable of explaining some hitherto not sufficiently explained phenomena, respecting the order of succession, in which different kinds of lava have been erupted. The Natural System of Volcanic Rocks of Richthofen, as generalized by Prof. Judd, asserts that, since the Kocene period, the earliest erupted rocks have been of an intermediate type, as regards the proportion of silica which they contain and their specific gravity. These were succeeded by more silicious lavas of less specific gravity; and finally followed by basaltic lavas, with the lowest proportion of silica and highest specific gravity. It is obvious that, if the couches of molten rock had been erupted in the order of their super- position, in which they must necessarily lie, those of inter- mediate specific gravity ought to have followed, instead of preceding, those of less. Our theory accounts for this circum- stance. The more highly silicious rocks being solidified in the crust, were underlain by a magma of intermediate character, and, during the movements which raised the mountains and depressed the roots, this made its appearance at the surface as lava of intermediate type. Subsequently the depressed siliclous crust was melted off, and underflowing the crust, the next eruptions were more silicious. When this layer, a thin one, had been erupted, or at distances from the ranges to which it had not extended, the denser substratum followed next in order, producing basaltic lavas. The theory of mountain roots also accounts for the fact mentioned by Prof. Judd, that the ejections in regions, lying upon opposite sides of a mountain chain, appear to have come __ COMIC OOOKS.EO Al MM cc el —___.-§- #&___—ts——t CS ts Ba in|