Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/202



The penetration of the wood by the siliceous matter is sometimes compleat; sometimes one extremity of the same fragment is thus petrified while the other remains in a ligneous state. The oak, the holly, and the hazel appear to have been the trees thus affected.

A phenomenon occurring in some caverns near the Black Rock, on the south of Church Bay in the isle of Rathlin, may be properly referred to this article.

These caverns (four in number) although excavated in the basaltic rock and at a point remote from any calcareous formation, are yet invested with calcareous stalactites, depending from their roofs, and by their droppings upon the floor depositing a crust of about an inch in thickness.

This circumstance appears worthy of attention, since the calcareous matter seems evidently, from the situation of the caverns, to have been derived from that which enters as a chemical ingredient into the composition of the basaltic rock, separated from the mass and deposited in its present situation by the percolation of water which the rain or springs must have furnished.

It proves therefore the permeability of basalt which has sometimes been denied, and gives countenance to the opinions of those who consider the nodules of calcareous spar and zeolite, occurring in the amygdaloidal varieties, as the results of an infiltration which has gradually filled up what were once vesicular cavities; it is remarkable that the substances so occurring are such as the chemical constitution of the matrix would qualify it to afford by a similar process; and in the instances above described, that very process may be detected passing under our immediate observation.

A vesicular variety of basalt, of which the pores contain water, occurs at Ballylaglan on the north of Coleraine. Dr. Richardson has mentioned it as a proof of the aqueous origin of basalt, believing