Page:The parochial history of Cornwall.djvu/39

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xxxi
GEOLOGICAL INTRODUCTION.

wall. It is there proposed to give these rocks, whenever they possess well-marked characters, distinct names, and not to refer a great variety of rocks to the vague and indefinite genera of clay-slate and greywacké. An accurate and more ex-tended nomenclature would have rendered the following notices much more intelligible.

In describing the general features of Cornwall, we must not omit to mention the metalliferous veins the great source of its commercial prosperity and the channel through which much curious information has been obtained, concerning the structure of the earth. These veins traverse indifferently both the granite and the slate, but are most abundant in the latter rock, in the vicinity of the granite. The general direction of the tin and copper veins (or lodes, as they are provincially called) is nearly E.N.E. and W.S.W., and they are crossed by another system of veins, nearly at right angles, which are not commonly metalliferous, and when they do contain ores these are often of lead, antimony, silver, and other metals. This is not however a general rule, for in the parish of St. Just, Penwith, the tin and copper ores occur in the cross veins. The course of the veins is not straight, but they are always more or less undulating, both in their direction and in their dip or underlie. Various interesting phenomena result from their meeting with and intersecting each other, known to the miner under the names of heaves, throws, and slides. This subject is replete with curious facts, but would require considerable space for their enumeration; the reader must