Canterbury and Westland. 339 



it stands vertical, although its course is a little tortuous and wavy. 

 I add a faithful sketch of the locality on Plate, of sections No. 6, from 

 the pencil of Mr. E. Dobson, C.E., from which it will be observed that 

 this dyke forms a conspicuous object in the landscape. The first 

 owner of the ground used it for a wall, and has broken a doorway 

 through it. It consists of a hard tabular basalt with crystals of horn- 

 blende and labradorite, and sounds like a clinkstone when struck 

 with the hammer. The tabular arrangement is parallel to the 

 flow, with no cross divisions at right angles to*it, except on both 

 sides, where the rock has a polygonic structure (see sketch on the 

 same plate), the horizontal divisions being four to six inches deep. 



Another dyke of a similar character, reaching to the very summit of 

 the caldera wall, is the one in which Mr. Fred. Thompson opened up a 

 quarry many years ago, and which afterwards has been worked by Mr. 

 Ellis. This dyke, having a direction of nearly west and east and 

 pointing towards Little Quail Island, is about 18 feet thick, and like 

 many others, is not quite vertical, having a dip of 85 deg. towards the 

 north. It stands several feet above the ground, forming quite a rocky 

 wall, and consists for the first 20 feet from its summit of a grey 

 porphyritic rock full of crystals of labradorite and basaltic hornblende. 

 It is a handsome building stone, being much liked in Christchurch, and 

 amongst other buildings it has been used in the erection of the Bank 

 of New Zealand, and for ornamental work in the Provincial Council 

 Chamber. On the southern side, the dyke has a tabular structure 

 for 6 feet 9 inches, and on the northern for 5 feet 6 inches, 

 the centre consisting of a homogeneous mass, which can be quarried 

 frequently in blocks of large size, indeed as large as 10 feet by 6 feet. 

 In quarrying downwards, the rock gets giadually darker in the 

 oentre, and after a short distance assumes the character of a black 

 dolerite, containing crystals of labradorite, augite, and hornblende, 

 and is of such hardness that it is useless for building purposes ; the 

 sides, however, still continue for some distance lower down to consist 

 of the same greyish dolerite, of which higher up — to use a quarry man's 

 expression — the heart had been formed (see section, plate 6). It is 

 thus evident that the sides and upper portion, by cooling more rapidly 

 and not being subjected to so great a pressure, could assume a more 

 porphyritic texture than the centre of the dyke, resembling the 

 basaltic rock of which the main mass of the larger lava-streams is 

 formed. 



