Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 1.djvu/414

 410 movements, and to convert them into receptacles of metallic ores, accessible by the labours of man. The greater part of metalliferous veins originated in enormous cracks and crevices, penetrating irregularly and obliquely downwards to an unknown depth, and resembling the rents and chasms which are produced by modern Earthquakes. The general disposition of mineral veins within these narrow fissures, will be best understood by reference to our first Section. (Pl. 1. Figs. k l.—k 24.) The narrow lines which pass obliquely from the lower to the upper portion of this Section, represent the manner in which Rocks of various ages are intersected by fissures, which have become the Receptacles of rich Treasures of Metallic Qre. These fissures are more or less filled with various forms of metalliferous and earthy minerals, deposited in successive, and often corresponding layers, on each side of the vein.

Metallic Veins are of most frequent occurrence in rocks of the Primary. and Transition series, particularly in those lower portions of stratified rocks which are nearest to unstratified crystalline rocks. They are of rare occurrence in Secondary formations, and still more so in Tertiary strata.