Correspondence — Prof. T. G. Bonney. 575 



THE FOLIATION OF THE LIZARD GABBRO. 



Sir, — Students of petrology will, T am sure, feel indebted to my 

 friend Mr. Teall, no less for his valuable paper on the Lizard Gabbro 

 than for the admirable Plate with which it is illustrated. He has 

 stated very clearly the reasons in favour of ascribing the peculiar 

 rock-structure, there described and depicted, directly or indirectly to 

 mechanical influences. In this it is quite possible he may be right. 

 But, inasmuch as there seems to me some danger at the present 

 time of overestimating the part played by mechanical agencies in 

 producing the crystalline schists — a part which hitherto undoubtedly 

 has been much underestimated — a danger in short of supposing that 

 a truth is " the truth, the whole truth, and nothing but the truth," 

 I venture to put a few words on record, not so much by way of 

 protest or criticism, as for the purpose of showing that in Mr. Teall's 

 proposed solution of this puzzle there are difficulties on which he 

 does not dwell, so that it ought at present to be regarded as an 

 hypothesis on its probation, and should not be quoted (as I feel sure 

 it would otherwise be) as an undoubted fact. My remarks, be it 

 understood, must be regarded as directed, not against the general 

 principle, of which this Lizard gabbro would be a particular case, 

 but against the application of the principle to this case. 



The difficulties which I feel may be thus stated. If the foliation 

 of the Lizard gabbro is the result of earth-movements acting on the 

 rock after it became solid, these movements, having resulted in 

 effects of an exceptional character, should have been of exceptional 

 importance — that is to say, the whole district should bear the impress 

 of the same earth-movement that has foliated the gabbro. Mr. Teall 

 states indeed : "that the Lizard district has been profoundly 

 affected by earth movements is apparent on every hand." Speaking 

 for myself, I should prefer to read 'considerably' for 'profoundly,' 

 so far as regards the earth-movements to which we could ascribe the 

 foliation of the gabbro. I am well acquainted with three large 

 separate areas of country which have been ' profoundly affected by 

 earth-movements' since their rocks became solid, and I do not find 

 a parallel to them in the Lizard, except perhaps at the extreme south 

 and near the great boundary fault on the north. There are un- 

 doubtedly numerous dislocations (as I have pointed out in one of my 

 papers on the district) ; there are endless wrenches, slips and nips ; 

 but there are few signs of a great compression such as may often be 

 traced through whole regions of crystalline rock almost as surely 

 (when the key to it is once found) as a slaty cleavage in one of sedi- 

 mentary rock. This foliation of the Lizzard Gabbro, as Mr. Teall 

 truly says, comes in and disappears, even in the thin veins, in appa- 

 rently the most arbitrary manner. This of course is perplexing in 

 any theory of its origin. I only quote it to show that our difficulties 

 are so far not diminished by the new one. Again, the foliation often 

 occurs where the neighbouring rocks show little or no signs of 

 material disturbance. I do not say that the contrary is not some- 

 times the case, but near the Balk (or Pen Voose), and to the north 



