Col. McMahon — On the Lizard Gahbro. 77 



the retention of heat (owing to vai-iations in the conducting power 

 of the surrounding rocks), more in some spots than in others ; and 

 thus some parts of the dyke might become more fused than other 

 portions, and might lose the foliated structui'e previously imparted 

 by the shearing motion. The heat thus generated, or retained, would 

 account for the assumption of a granitic structure in portions of 

 a dyke, but not for the foliation of tlie other portions. 



Mr. Teall considers " that the most convincing proof that the 

 foliated structure is the result of movement after the consolidation 

 of the rock is furnished by its relation to fault planes. A rock must 

 necessarily be solid before it can be faulted." But we learn from 

 Mr. Teall's paper tliat thei'e must have been a lengthened series of 

 earth-movements at the Lizard ; thus he speaks of the " super- 

 position of the effects of distinct movements " ; the fractures there- 

 fore, to which he alludes, may have taken place after the consolida- 

 tion of the rock; and after the foliation had been produced. Do all 

 the faults in the gabbro present foliated margins ? I gather from Mr. 

 Teall's article (read with Professor Bonney's paper in Q.J.G.S. vol. 

 xxxiii.) that this phenomenon is principally to be observed at the 

 junction of the gabbro with other rocks. But even if some of the 

 faults in the gabbro do present this phenomenon, I do not see why, 

 in those exceptional cases, the faulting should not have taken place 

 before the complete consolidation of tlie gabbro. If the fault ran 

 through the rocks above and below the gabbro, the gabbro would 

 surely have divided in the line of the fault as readilj^ as a layer of 

 jam between slices of sponge cake does when the cake is broken. If 

 the gabbro was not completely rigid where thus divided, the shear 

 might have produced a foliated structure in the portions exposed to 

 the teai'ing effects of the rupture. 



In conclusion, I would invite the attention of the readers of the 

 Geological Magazine to the sketches of two foliated gabbro veins 

 in the Lizard serpentine drawn by Professor Bonney (figs. 2 and 4), 

 which will be found at pp. 893, 897, of his paper in vol. xxxiii. 

 (1877), Q.J.G.S. Professor Bonney, in his letter published in the 

 December Number of your Magazine, tells us that the serpentine is 

 " free fi'om all indications of mechanical disturbance." This being 

 so, it seems physically impossible that the foliation of the gabbro in 

 the veins depicted in his paper can be due to " shearing movements " 

 acting on a " solid rock." Furthermore, it will be observed that the 

 foliation in one of the veins (fig. 2) not only runs with the two sides 

 of the vein, but also with the boundarj'^ of the terminal end {vide the 

 illustration). Professor Bonney tells us that the foliation runs 

 "roughly parallel to the longer sides of the vein, except towards the 

 top, where it tends to become parallel to the upper surface." This 

 is what I should expect to see if gabbro in an imperfectly con- 

 solidated, or semi-plastic condition were forced into, or through, 

 an opening in the serpentine rock ; but it is, to me, totally in- 

 comprehensible on the hypothesis of " shearing motion parallel with 

 a fault plane." 



