Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/88

 could not be less than 60 or 70 feet. This remark is not mere matter of curiosity, it leads to geological inferences not unworthy of regard, as will be presently seen. Their direction is almost invariably either vertical or slightly inclined from the plumb, and they present therefore a perpetual parallelism along the coast. They are equal throughout, and never ramify, and although they vary from 5 feet to 20 in breadth, they more commonly are of a dimension not exceeding 10 feet. It is well known that the permanence of trap veins is sometimes less, sometimes greater than that of the surrounding rocks. Hence they sometimes project like walls, while at others their ruin produces fissures or caves. This latter effect has taken place here, marking a great tendency to decomposition in these veins, since the including rocks do not seem to be of a very durable nature. The depth to which they have been excavated is often very considerable: in the case, of the spar cave including the external fissure, it cannot be less than 250 feet. In consequence of this wasting it happens that the intermediate cliffs which remain, have, as I have noticed in the general description, the appearance of the ends of walls; and as they also sometimes yield and fall away behind, in such cases they present the appearance of insulated square pillars of masonry, the resemblance being rendered perfect by the channelled marks of the strata formerly described. These veins are often stratified, or more properly speaking, laminated in the direction of their length. They are generally formed of a bluish black basalt; at times they are porphyritic, or vary in other ways, which it is unnecessary to describe. I observed in one nodules of prehnite, the only occasion on which I have found that mineral in veins of trap. In another I found a second vein, holding a serpentine course through the first in somewhat parallel direction, and readily distinguished by being formed of a much more black