392 Transactions. — Geology. 



Like the dykes in the viciuity, it is divided into polyhedric form — at 

 right-angles to the cooHng surfaces, and is also jointed parallel to the direc- 

 tion 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 speci- 

 mens 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 wast- 

 ing 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 

 occm'S. 



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 north- 

 ward 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 West- 

 land," 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 ca^D ; and also to 

 examine, with the microscope, thin slices fi'om 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 sub- 

 jected 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 maybe assumed 

 that a column 1 inch square and 10 feet high weighs 120 Hbs ; thus suppos- 

 ing the dj^ke-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- 



