Col. McMaJion — On the Lharcl Gahhro. 75 



result of pressure " acting " after the consolidation of the rock ; " or, 

 as he puts it shortly in another place, the foliation is "a secondary 

 structure due to earth-movements acting upon the solid rock." [The 

 italics are mine.] 



Mr. Teall does not expressly state, in his interesting and valuable 

 paper, how the pressure acts so as to produce the results described ; 

 but we are told that at Pen Voose " massive gabbro passes over into 

 gabbro schist at a fault plane," and "the foliation in the gabbro is 

 such as would be produced by a shearing motion parallel with the 

 fault plane." 



Are we to understand that foliation has been produced by mere 

 mechanical crushing, and consists of the rearrangement of minerals 

 already formed sliding over each other's backs — a sort of mineralogi- 

 cal leap-frog — in consequence of a " shearing motion parallel with 

 the fault plane ; " or are we to understand that the " shearing motion" 

 was transmuted into heat, and the heat so produced resulted in the 

 fusion and subsequent recrystallization of the component minerals ?' 



On either supposition I shall be glad to know how the foliation 

 of the " veins and dykes of gabbro " to the north of Coverack, and 

 the " small dykes of gabbro in the serpentine near Karakclews," is 

 to be accounted for ? Do all these veins and dykes concur with fault 

 planes, or do they ramify in all directions in the usual manner of 

 intrusive veins ? If they coincide with fault veins, how is it that 

 they are not always foliated ? Mr. Teall states that the Coverack 

 veins, and the small dykes near Karakclews, are " often foliated " 

 (which implies that sometimes this is not the case) ; and further, 

 that " not seldom one may see the transition from massive gabbro to 

 gabbro schist taking j)lace in the space of a few inches." If the 

 veins and dykes in question coincide with fault planes, this occa- 

 sional failure of "shearing motion" to produce foliation seems to 

 need explanation. On the other hand, if some of the foliated veins 

 and dykes do not coincide with fault planes, I fail to see how the 

 " shearing " hypothesis can account for the foliation. 



Again, if the foliation is accounted for on the supposition that 

 the shearing motion developed heat, and recrystallization took place 

 on the subsequent cooling of a rock in which molecular motion had 

 been set up by the heat so developed, I do not at present understand 

 how masses of gabbro " more than a hundred yards in width " 

 (p. 487) have become, " with a few local exceptions, foliated 

 throughout " their " entire mass." If the heat produced was suffi- 

 cient to fuse the rock, why was its structure, on reconsolidation, not 

 granitic, as it presumably was on its first consolidation, and as the 

 dykes, sheets, and veins of ordinary eruj)tive rocks are? 



Most geologists with field experience will doubtless be able to 

 point to numerous instances of such eruptive dykes, and sheets, that 

 have consolidated under enormous pressure, but which, notwith- 

 standing, do not show a trace of foliation. 



The above, I think, are difficulties which need clear explanation 

 before the hypothesis can be accepted that pressure acting on a solid 

 rock is sufficient to account for the foliation of the Lizard gabbro. 



