Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/73

 cannot be distinguished, and which ought therefore to be considered a basalt, since no other criterion can be established between the two, the variable proportions of hornblende and felspar alone admitting of no better or more defined limit than this. More generally in these mountains the greenstone assumes a large grain and very coarse texture, and in some cases the separate substances exhibit crystals of a quarter of an inch in dimension, while the hollows which are found in the rocks are sometimes occupied by detached crystals of hornblende. The felspar in these examples is often of a greenish hue. This rock appears remarkably permanent, showing few traces of waste or decomposition of the surface, and it is of this particular variety that the rugged summit of Garsven is composed. Among the finer grained varieties a remarkable kind is found on the borders of the romantic lake Coruisk, where it lies in detached blocks rolled down from the surrounding mountains. It is honey-combed into large cavities, which allow the hand to enter deeply within them, while at the same time the surfaces are almost as flesh as if recently broken, showing none of that rusty stain which attends the decomposition of greenstones in general. These blocks are extremely sonorous, and, notwithstanding their thickness, they ring when struck, with a sound as great as, and precisely similar to, that of a thin vessel of cast iron. In other places the same rock is found studded over with large detached protuberances resembling pedunculated fungi, or huge nails driven into it. The last variety which I shall mention is found in the same place, and it is the most remarkable, since it presents modification of trap hitherto undescribed. It forms a great portion of that naked and wild surface which I have already described in the general account of this spot, being disposed in enormous smooth inclined beds extending, without fissure or trace of decomposition