Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/405



What the situation of the present vein is with respect to the horizon I have no means of knowing, nor is it a matter of much moment, as it is tolerably evident that the strata which are now visible on the surface of the earth do not always lie in the positions in which they were formed. There can be no question that slides of this nature are of different æras, as we may often observe a succession of them in which the first has been shifted by a subsequent one. Whether those represented in the present drawings are referable to one or to different periods, there is no appearance to decide, though their uniformity would lead us to suppose that they were all produced by a single cause and at one time.

Some unnecessary doubts appear to have been entertained by geologists respecting the formation of those smaller veins which have their origin and termination in the rock where they are found, and which have no communication from without. While one party has denied their posterior origin to the rock in which they are contained, and asserted that they were of “ contemporaneous formation” with the containing parts, another has had recourse to an igneous hypothesis for the purpose of solving a difficulty of which the explanation appears abundantly simple. It is universally known that many rocks contain much water in a state of intimate mixture, or perhaps combination, which they are subject to lose on drying or by exposure to the air. From this cause they contract and form fissures. Similar fissures occur from the ordinary subsidences and fractures of parts either ill supported or subjected to external violence. Such cavities being formed the process of infiltration commences. The water existing in the rock percolates into the cavities, sometimes forming crystals, and sometimes filling the cavity with a solid mass of the matter which it held in solution. When silica exists in the rock, veins of quartz are thus formed; when lime, calcareous spar.