Like the dykes in the viciuity, it is divided into polyhedric form—at right-angles to the cooling surfaces, and is also jointed parallel to the direction of flow. The rock forming the cap comprises many different qualities, varying from a workable stone, like No. 13, to a hard slaty rock, as shown by specimens Nos. 14 and 15. It was evidently of much greater extent, the existing portion being the central mass, which has survived the general denudation the outer portions having been undermined from time to time by the wasting away of the softer underlying rocks. At the junction of the cap with No. 11 both rocks appear to be slightly altered, and at some places, especially on the west side, a hard slaty selvage occurs. The most striking feature in the case is the great change which takes place from the soft freestone (specimen No. 8), which occurs to the northward of the cap, to the hard dark rock in the cap itself. No. 8 is like the brown stone at present being worked in Thompson's quarry on the opposite side of the valley; whereas the stone of the cap is very similar to that occurring in the dykes which crop out along the Sumner road, between Heathcote ferry and Sumner. Dr. von Haast, in Chapter XII. of his “Geology of Canterbury and Westland,” mentions the fact that the chemical constituents of dyke stones taken from different localities, vary very considerably, although their appearance is in every respect the same; but in this case the stone varies in appearance to such an extent that it is difficult to believe it to be part of the same dyke without personally tracing out the continuity; and doubtless the chemical composition varies as much as the general aspect. It would be very interesting to analyse a set of specimens, taken in ascending order, from different parts of the dyke and cap; and also to examine, with the microscope, thin slices from the same places. These two series of observations would throw much light upon the chemical change and action of the rock, both when under and when free from pressure. In this case it would appear that the rock is hardest when it was subjected to the least pressure. The enormous pressure the dyke rock must have been subjected to when being forced up the chasm, is readily seen by estimating the weight of a column of stone an inch square. For sake of comparison, it may be assumed that a column 1 inch square and 10 feet high weighs 120 lbs; thus supposing the dyke-stone to be in a fluid state, the pressure 10 feet below the surface would be 120 lbs. per square inch; at 100 feet below the surface, 1,200 lbs.; and at 1000 feet, 12,000 lbs. Now even assuming that, when in a state of ebulition, the action of the entangled gases would relieve a cer-