v.— G E L G Y. 



Art. LV. — Description of a remarkable Dyke on the hills near Heathcote. 



By A. D. DoBsoN. 

 [Bead before the Philosophical Iiistitute of Ganterhury, \st April, 1880.] 



On the west side of the Heathcote valley a remarkably sharp peak forms the 

 crest of the hill, the almost vertical sides of which are in marked contrast 

 with the rounded slopes immediately below it. 



This peak is formed by the outcrop of a volcanic dyke, which, flowing 

 over the edges of the original chasm, forms a cap about 550 feet long, 

 200 wide, and from 70 to 80 feet high. 



On the north side of the peak the dyke is first visible about 90 feet below 

 the lower edge of the cap, at which point it is just traceable as a narrow 

 band, chiefly noticeable by a hard selvage on the west side passing through 

 a coarse-grained Porphyritic dolerite, of which No. 1 is a specimen. 



Proceeding upward the dyke rapidly widens, and, 20 feet above the point 

 where it first appears, it is 10 feet wide, and well defined. Specimens No. 

 2 and 3 are from the dyke at this point ; No. 2 bemg from the selvage on 

 the west side, and No. 3 from the centre. A little higher up the stone be- 

 comes more compact, as shown by specimen No. 8. 



The dyke here passes through about 20 feet of basaltic tufa, shown by 

 specimens 4, 5, and 6. To the westward this bed becomes intercalated by 

 bands of a harder stone, specimen No. 7 ; changing further westward into 

 or replaced by No. 7a. Above this up to the cap, a height of about 55 feet, 

 the dyke can be traced, but indistinctly, owing to the superincumbent mass 

 of loose rock. 



The rocks passed through are hard porphyritic basalt, about 30 feet 

 (shown by specimens Nos. 9 and 10), and a soft dolerite lava, about 25 feet 

 (specimen No. 11). This is the rock on which the cap rests. The junction 

 of the dyke and cap at the north end is obscured by the mass of loose rock 

 lying on the northern slope, but at the southern end it forms the crest of the 

 spur, is about 20 feet thick, and can be readily traced to the top of the 

 crater-well. At this end it is mostly of a much harder material than at the 

 north end, being very similar in character to the rock forming the cap, but 

 in places it passes into a soft freestone. 



