Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/450

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Dr. Mac Culloch on the Geology of

nothing to add in explanation of them; they merely suffice to prove, that these substances like the schistose rocks, have been in a state which admitted of bending; the capricious and intricate contortions visible in some of the figures (which were drawn with great care) not admitting of any fissure of this form to be subsequently filled either by injection or infiltration. One of the figures, Pl. 31*, left hand figure at top[errata 1], is perhaps particularly worthy of notice, on account of the distinctness with which it demonstrates the action of partial forces, by the remarkable difference of curvature occurring in two veins so nearly approximated.

Burnt Island.

A specimen of a compound vein is before the Society, which traverses the well known amygdaloid of this shore. This vein consists of a highly crystalline brown limestone, mixed with a ramified compact earthy black basalt. These substances are so intermingled, that it is scarcely possible to conceive that the basalt is posterior to the limestone, or that a basalt vein should have found its way into a vein of limestone without also traversing the amygdaloid. It is probable that the whole vein is of simultaneous formation, and that the substances have separated from each other in consequence of those obscure chemical affinities which regulate the crystallizations of compound rocks. This however is a subject deserving of further investigation, as, if it is admitted, it will prove that the same cause, whatever that be, may preside over the formation of basaltic veins, and those veined limestone deposits in trap rocks, which are called calc sinter, and are held to arise from the posterior effects of a watery infiltration.

In the same rocks a singular circumstance accompanies the imbedded calcareous nodules. Their surfaces are occasionally marked


  1. Original: was amended to Pl. 31*, left hand figure at top: detail