Major-Gen. McMahon — Culm- measures at Bude, Cornwall. 115 



to leave out of consideration cases in which shearing is the alleged 

 cause of metamorphism. Shearing would open up a very wide 

 branch of inquiry, and as the Bude rocks do not exhibit cleavage, or 

 foliation, or mineralogical changes that could be attributed to shear- 

 ing, it will simplify matters to eliminate this branch of dynamic 

 metamorphism from the discussion. 



There remains the question whether pressure, apart from shearing, 

 is capable of setting up molecular activity of such a character that 

 the chemical constituents of a rock rearrange themselves in the 

 manner implied by the use of the term metamorphism ? 



In this connection I do not think I can do better than quote from 

 the highly interesting opening address of the President of Section G- 

 at the last meeting of the British Association. 1 After alluding to 

 the mobility of the atoms or molecules of solid bodies, and after 

 giving an account of Spring's experiments, Mr. Anderson goes on to 

 say: "The same movements and changes have taken place and are 

 still going on in Nature's laboratory. During the countless ages 

 with which geology deals, and under the enormous pressures of 

 superincumbent masses, stratified sedimentary rocks become crystal- 

 lized, and assume the appearance of rocks of igneous origin, and not 

 only so, but rocks of whatever origin, crushed and ground to pieces 

 by irresistible geological disturbances, reconstruct themselves into 

 new forms by virtue of the still more irresistible and constant action 

 of molecular forces and movements." 



The student must not allow his mind to be mystified, or carried 

 away from the real point at issue, by such phrases as " the 

 countless ages with which geology deals." The Alps and the 

 Himalayas had their birth in Tertiary times, and if the rampant 

 metamorphism of Himalayan rocks is to be referred to the dynamic 

 throes of mountain-making, those throes were, geologically speaking, 

 of yesterday's date, and we cannot indent on " the countless ages 

 with which geology deals " to impart a poetic and occult glamour 

 to the prosaic details of dynamic force exerted so recently as the 

 Middle Tertiary period. 



One other remark I would make, and that is, that laboratory 

 experiments, like Spring's, however instructive and valuable, hardly 

 run on all fours with the conditions that exist in nature. Mr. 

 Anderson, for instance, with his eye on Prof. Spring's pestle and 

 mortar, speaks of rocks, like Spring's powders, being "crushed and 

 ground to pieces by irresistible geological disturbances " before 

 they began to " reconstruct themselves into new forms by the still 

 more irresistible action of molecular forces and movements." I 

 am not sure whether Mr. Anderson considers this grinding to powder 

 an essential part of the process in every case ; if he does not, Spring's 

 experiments may be put out of court at once ; if he does, this as- 

 sumption, it seems to me, is likely to prove fatal to the theory. In 

 the Himalayas we have to deal with outcrops of foliated granites, 

 not to speak of other crystalline rocks, ten miles thick and hundreds 

 of miles long, and can any one, I would ask, believe that these 

 1 Nature, September 19th, 1889, p. 510. 



