﻿Vol. 64.] BASIC i:n'teusiok op bartestree. 503 



with pale spots and patches, which partly consist of epidote and, 

 as was noted by Murchison, have a superficial resemblance to 

 amygdules. The alteration is strongly marked, right up to the edge 

 of the cutting at the south-western end near the trap ; but farther 

 north-eastwards, where the cutting widens somewhat, unaltered 

 red sandstone is exposed. 



Phillips {loc. jam cit.) noticed the occurrence of rude columnar 

 jointing in the sedimentary rocks adjacent to the dyke — the columns 

 being arranged at right angles to the margin of the trap, and 

 extending for a distance of several feet. This is scarcely noticeable 

 at the present time. 



III. General Relations of the Intrusive Rocks. 



As regards the nature of the intrusion, the earlier writers refer 

 to the rock as a greenstone ; Murchison states that it is composed 

 of hornblende, olivine, and felspar. The Rev. J. D. La Touche, who 

 gives a full account of its field-relations, describes it as a green- 

 stone or diorite. A microscopical examination, however, shows that 

 the rock is clearly basic. It is not, however, a simple uniform 

 intrusion, but is composed of several allied though difi'ering types 

 of dolerite and basalt. A comparatively slight examination soon 

 shows that, while the main part of the dyke is a compact fine- 

 grained basalt, doleritic material prevails near the southern margin 

 and to a less extent near the northern.^ There is often some 

 difficulty in distinguishing between the two rock-types in the field, 

 especially as the basalt when weathered tends to resemble the 

 dolerite ; and a more detailed examination, while confirming the 

 fact that the central part of the dyke is predominantly basaltic 

 and the marginal part predominantly doleritic, made it clear that 

 the two rock-types are intimately intermingled, patches of basalt 

 occurring in the predominantly-doleritic portion and vice versa. 



IV. Description of the Intrusive Rocks. 



Owing to the very considerable amount of variation in the 

 character of the rock in different parts of the intrusion, a somewhat 

 detailed description will be necessary. 



(A) The Dolerites. 



Two distinct types may be recognized : — 



(I) Along the southern margin of the intrusion a rather fine- 

 grained dark yellowish-green doleritic rock, having the low specific 

 gravity of 2-65 and containing 45*45 per cent, of silica, prevails. 

 Excellent junction-specimens with the Old Red Sandstone are 

 readily obtainable from a thin layer of dolerite which covers the 

 Old Red forming the southern wall of the quarry nearest the trap. 



1 The Rev. J. D. La Touche (Trans. Woolhope Nat. F.-C. 1891, p. 167) says 

 that the central part is more coarsely crystalline than the marginal part. 



