Penny Blood: Serial Sensation and Working-Class Entertainment — page 301: what you’re looking at

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

284 SUMMARY. [cH. xxII. regard to the original addition of volume, which the presence of the water could contribute. Changes in the earth's rotational velocity may have altered the oblateness, aud compressed some parts of the surface and stretched others; but there does not appear to be any indi- cation, in the arrangement of mountain chains, of their having been connected with such a mode of action. XV. We have thus examined two probable causes of com- pression of the earth’s surface, originating in contraction of the interior, namely, cooling, and extravasation of water-sub- stance from beneath the crust. Neither of these appears to be adequate to produce the amount of compression which exists. No other cause of contraction of the globe suggests itself. We next look about us for a possible cause of extension of the surface, which, it 1s obvious, would, equally with contraction of the interior, produce compression. The numerous more or less vertical dykes of igneous rock lead us to enquire, whether the rending of the fissures which they occupy, may not be indica- tive of the extension of which we are in search. The pressure of the crust upon the fluid substratum is about 10066 tons upon the square foot. If the water-substance from the magma were to escape into a crack in the under side of the crust, it would exert this pressure towards rending it wider; and so long as the vapour or gas was supplied with sufficient rapidity, it would prevent the magma from rising into the chasm so formed. When the rent reached the surface, the vapour would rush forth and be followed by the magma itself, now appearing as lava; and thus a volcano would be established. But this would be an exceptional occurrence. It would be only here and there that the vapour would escape at the surface, because its doing so at one point would relieve the internal pressure for a long distance. The above-described process appears to be the mechanism requisite to establish a volcanic vent, and agrees well with the series of earthquakes which usually for a long while precede an eruption, and very often occur without any eruption at all. Nevertheless permanent elevation of the tract is said to be a common result of their action. COLA @ NOOKS,. ECO A |