222 Major- Gen. McMahon—The " Culm " at Bade. 



VI. — The Culm-measures at Bude, North Cornwall. 



By Major-General C. A. McMahon, F.G.S. 



r Y paper on the Culrn-measures at Bude has elicited two com- 

 _H| munications, from Messrs W. Maynard Hutchings and Alfred 

 Harker, on which I desire to offer a few comments. 



I take this opportunity of saying that I am extremely glad to 

 find that the conclusion at which I arrived regarding the impotence 

 of pressure alone to produce metamorphism is in accord with the 

 published views of my friend Mr. J. J. Harris Teall. " Pressure " 

 alone, he states, in a footnote at p. 410 of his valuable work, British 

 Petrography, " produces no effects on rocks — work must be done 

 upon rocks before change takes place." This footnote escaped my 

 memory at the time of writing my paper, or I should have called 

 attention to it. 



I now turn to Mr. W. M. Hutchings's letter. He points out that 

 he collected two specimens of Bude rocks "a few inches apart," 

 and after " a close study of very thin portions of slides under high 

 powers," he found acicular crystals, and dark rods of rutile, " per- 

 fectly distinct from any bits of that mineral which may have come 

 from older rocks." 



I did not notice any rutile in my slices ; but whilst I readily 

 accept Mr. Hutchings's assurance that high powers applied to 

 specially thin portions of his slides revealed the presence of rutile, 

 I fail to see how this fact proves that metamorphism has been set up 

 in the Bude rocks. 



Mr. Teall has shown l that rutile needles occur abundantly in 

 rocks so absolutely untainted with a suspicion of metarnorphism as 

 clays : moreover, Mr. Dick found rutile in the Hampstead Sands. 2 



Unless Mr. Hutchings can adduce cogent evidence to prove that 

 the rutile needles in his Bude specimens were formed after the Bude 

 beds were laid down, I think the natural inference is that they were, 

 like the needles of this mineral found by Mr. Teall in his clays, and 

 the crystals in the Bagshot Sands of Hampstead Heath, floated to the 

 spot with the rest of the fine material of which these beds are com- 

 posed. Mr. Hutchings, it is true, alleges that his slides contain 

 " secondary sericitic mica " ; but he does not favour us with any 

 evidence to prove either that the mica is sericite or that it is of 

 secondary origin. 



Bosenbusch says, "the optical behaviour " of sericite " is exactly 

 the same as that of muscovite ;" 3 and "it is probable that substances 

 of different composition are included under sericite." 4 In J. D. 

 Dana's Third Appendix, sericite is said to be " a massive muscovite, 

 as shown by Laspeyres, who explains the varying results of earlier 

 investigations by the greater or less impurity of the substance ex- 

 amined." 



Mr. Hutchings also relies on the presence of crystals of tourmaline, 



1 Min. Mag. vol. vii. p. 201. 



2 Nature, vol. xxxvi. p. 91 ; and Brit. Petrography, pi. 44, fig. 4. 



3 Microscopical Physiography, by Iddings, p. 2t>5. i Ibid. 



