Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/56

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Dr. Berger on the Isle of Man

II. Compound Rocks not in Situ.

(a) Primitive Rocks.


I am aware but of two modes of explaining the existence of loose blocks of rocks that are spread over the face of a country, whatever may be the nature of the blocks themselves, either to suppose they are extraneous to the places where they now lie, or that, howsoever unconnected they may appear to be with the materials that surround them, they are nevertheless in their birth-place, and have been disintegrated in situ, covering and resting upon solid and continuous strata similar to themselves.

The only criterion perhaps applicable to the determination of this important question is the following. When by farther investigations we have found that the hidden and continuous rocks are similar to the blocks themselves, we may safely venture to say that the latter are indigenous.

I am inclined to think that several of the loose blocks I am going to describe; whenever they occur in any number, are in their birth-places.

On the beach towards Aire-point there are innumerable loose blocks of granite, one of which, rather large in its dimensions, I observed on Aire head at the elevation of 271 feet. They all in their characters differ but little the one from the other. The rock is a small-grained granite, composed of white felspar, quartz and black mica, with a few incidental plates of the latter that are white. In one of the specimens I collected, there is a rectangular piece one inch in length by half an inch broad, of very minutely grained granite with a predominance of mica.