Page:The parochial history of Cornwall.djvu/37

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xxix
GEOLOGICAL INTRODUCTION.


and at the surface are both subdivided near the middle, by the overlapping of the slate: the other two are small, but very interesting to the geologist.

It is worthy of remark that the fertility of these granitic groups gradually increases as they diminish in elevation; and it is a curious but not surprising coincidence, that the number of parish churches thereon follows the same order: thus on the eastern and most extensive tract of granite, near Launceston, there is only one church; on the next there are three; on the Redruth patch six; and on the Land's End granite no less than nine, within a space considerably less than that of the eastern tract.

Each of these insulated groups of granite is surrounded by schistose rocks, the layers of which, on all sides, incline from the granite at various angles, from 20° to 40°. Although these groups are thus separated from each other by the slate at the surface, yet it is the general opinion that they gradually approach beneath, until they are all united into one and the same mass the intermediate hollow spaces (the valleys, as it were, be- tween the granitic mountains) being occupied by the slate. It might, however, be maintained that the granite is imbedded in the slate, in large rounded masses, which would also account for the former rock underlying the other, within the limits of mining operations: and such an opinion would derive some support from the fact, that small insulated masses of granite in the slate are not of unfrequent occurrence.