76 Col. McMahon — On the Lizard Gahhro. 



In the mean while, I venture to offer another explanation for con- 

 sideration. 



After having studied what used to be called the granitoid -gneiss 

 of the Himalayas for some time, I came to the conclusion, from the 

 strati graphical, petrological, and microscopic facts observed, that 

 what is now called the gneissose-granite of the Himalayas was 

 erupted through lines of fracture in a partially consolidated condition. 



1 do not wish to apply that explanation to all cases, but I suggest " 

 a modification of it to account for the foliation of the Lizard gabbro ; 

 namely, that the latter was produced by " shearing motion " on the 

 rock after its consolidation had advanced considerably but was still 

 incomplete. This, it seems to me, would account for the observed 

 facts better than any hypothesis yet proposed for acceptance ; it would 

 account for the foliation being generally marked at the contact of the 

 gabbro with other rocks along joint planes ; it would account for any 

 signs of crushing in the gabbro that the microscope may reveal ; 

 and indeed for all the observed facts so far as I am acquainted with 

 them. The great depth to which the foliation sometimes extends 

 might, on this hypothesis, be accounted for either by supposing that 

 the " shearing motion " continued for a long time ; or, better still, 

 that it was repeated frequently at intervals of sufficient duration to 

 allow the marginal portions of the gabbro to cool and consolidate. 

 Very slight movements parallel with the fault-plane would be suffi- 

 cient, on the latter supposition, to produce foliation to a great depth. 



If the foliated veins and small dykes, alluded to b}^ Mr. Teall, do 

 not run with fault planes, the foliation might be explained on the 

 supposition that they were, like the gneissose-granite of the Himalayas, 

 intruded in a viscid, or semi-plastic condition, when the shearing of 

 the main mass of the gabbro along the fault planes was going on. 

 Traction action on an imperfectly consolidated mass would in this 

 case account for the foliation observed. 



The fact that in some of the dykes a rapid passage fi-om a foliated 

 to a granitic structure is observed, is of course a difficulty in the way 

 of any hypothesis ; but I suggest that on ray hypothesis it is capable 

 of explanation. In the first place, I think it might reasonably be 

 supposed that the cooling, and consequent consolidation, of the rock 

 proceeded more rapidly in some parts of the dyke than in others. 

 Eocks are rarely perfectly homogeneous ; we often find variations in 

 density, and hardness, within short distances even in some rocks 

 of apparently homogeneous composition — a difference not due to 

 weathering, but which weathering, and the agents of erosion, speedily 

 find out. Then the power of conducting heat must have varied much 

 in the bounding rocks. From these two causes the solidification of 

 parts of a dyke might have been in advance of neighbouring portions, 

 and have resisted the shearing motion more than the latter. The 

 portions in which solidification was most advanced might thus have 

 retained their granitic structure, whilst the less solid parts might have 

 yielded to the shear and have become foliated. 



In the second place, I think it possible that shearing motion might 

 result in the development, or what would come to the same thing, 



