ON THE CRYSTALLINE KOCKS OF TUK LIZAKD DISTRICT. 48Ii 



(2) The (lahhro. 



The principal mass of gabbro, as stated in a former paper, is 

 rudely oval in form, the longer axis measuring full four miles, and 

 the shorter a])out two. It rises in Crousa Down to a height of 

 nearly 300 feet above the sea, by which it is washed for a con- 

 siderable distance north of Coverack Cove. In the Survey map it 

 is represented as giving place to greenstone in the little cove opposite 

 to the dangerous skerries called the Manacles. This, however, is 

 hardly correct, for though dykes of the latter rock become rather 

 more frequent on this part of the coast, and perhaps ultimately 

 occupy as much space as the gabbro itself, that rock continues to 

 Porthoustock Cove, on the southern slopes of which it may be seen ; 

 though, as will be hereafter noticed, it does not, so far as we know, 

 descend to the water's edge. 



This mass of gabbro evidently throws off many veins on its south- 

 ern flank, which cut both the troctolite and the serpentine in Cover- 

 ack Cove. There is also the great dyke-like mass, nearly two miles 

 long and about a furlong wide, according to the Survey map, which 

 runs inland roughly in a JST. W. direction from the skerries of Carrick 

 Luz, and approaches at nearest within about a third of a mile of 

 the former mass. On either side, in Lankidden Cove on the east and 

 towards Compass Cove on the west, dykes are numerous, doubtless 

 in some way connected with it. They disappear in Kennack Cove, 

 but are found again about Enys Head, and then, after a consider- 

 able interval, at Polbarrow, becoming ultimately very numerous 

 around Pen Voose. It is, however, only at the two first-named 

 localities that the rock is found in masses of considerable size; 

 generally it occurs in dykes or veins (at most only a few yards, 

 and commonly only a few feet thick) which not seldom ramify and 

 terminate in veins sometimes less than an inch in thickness. 



The mineral composition of the rock and its changes have already 

 received full attention ; therefore it may suffice to say that in its 

 normal condition it varies from a plagioclase-olivine-augite (or 

 diallage) rock to a saussurite-hornblende rock, the last mineral being 

 partly actinolite, and always one of the distinctly green varieties. 

 It is impossible in this case to prove that olivine was an original 

 constituent, but inasmuch as it is present in certain masses, which 

 exhibit a transition from the normal gabbro to the ordinary saus- 

 surite-hornblende rock, there is no reason for supposing it to have 

 been originally absent from the latter. In the great mass at Crousa 

 Down the gabbro is often comparatively unaltered. In the dykes, 

 including the large one at Carrick Luz, it is generally more or less 

 altered. The olivine has usually disappeared, though occasionally 

 its position is indicated (^as is rather common at Coverack) by a 

 blotch of hematite ; the augite occurs as diallage, and every stage 

 of the change from this mineral into hornblende can be observed. 

 The felspar in like way passes gradually into saussurite ; in most 

 cases it appears to he less stable than the diallage, for a saussurite- 

 diaUage rock is common. The change to saussurite does not appear 



