Correspondence — Mr. W. M. Hutchings. 189 



But in any case, even though these specimens show that all the 

 Bucle rocks are not without distinct evidence of inetaniorphic action, 

 it is still true that the effect produced is not in anything like the 

 proportion we might expect, from the stresses endured by these beds. 



I am not able to follow some of the reflections which Major- 

 General McMahon bases on the supposed total absence of alteration 

 at Bude. 



Hallock's experiments (as quoted), and still more Hallock's con- 

 clusion from them, seem to be beside the mark. It is not generally 

 supposed that pressure is able to liquefy rocks, — quite the reverse in 

 fact, — and there does not seem to be any justification for saying that 

 " consequently " no chemical or mineralogical changes are to be 

 expected. 



Again, Spring's experiments are admitted to have proved that 

 pressure can produce chemical combinations and re-arrangements ; 

 and nothing that was done by " Professor Spring's pestle and 

 mortar " would be lacking in the intermixture of minute particles of 

 minerals in the fine silt of which these Bude rocks and similar 

 strata are largely composed. There is no call here for rocks to be 

 " crushed and ground to pieces by irresistible geological disturb- 

 ances." All the crushing and grinding has been done in the 

 gentlest and quietest way, and the resulting material has but to 

 lie and await the pressure. 



Whether pressure, with or without movement, is in itself 

 sufficient to intensely metamorphose sedimentary rocks, is another 

 question. 



And, if it is sufficient, there is still much room for inquiry and 

 speculation as to why it acts so comparatively feebly at one place 

 and so very intensely a few miles away, when, so far as can be 

 judged from the rocks, the feebler metamorphism has by no means 

 corresponded to feebler stresses. 



Newcastle-on-Tyne, W. Maynard Hutchings. 



March 10th, 1890. 



CONTORTION AND METAMORPHISM. 



Sik, — General McMahon's " Notes on the Culm-measures at Bude " 

 in the March Number of this Magazine (p. 106) form a welcome 

 contribution to the petrology of the district, and have a particular 

 interest as indicating the probable derivation of the strata in question 

 from the destruction of granitic rocks. The fact that the Culm- 

 measures are much contorted without having experienced any ap- 

 preciable mineralogical changes seems, however, to have only a 

 limited bearing on the general question of metamorphism by pressure. 



Adopting the familiar treatment employed by Thomson and Tait, 

 we may usefully resolve any system of strains into (i) a uniform 

 voluminal compression and (ii) certain shears. The term shear is 

 here used in its strict sense, viz. deformation apart from change of 

 volume, and it is evident that the varying amounts of shearing from 

 point to point within the mass express themselves completely in the 

 contortion of the rocks affected, faulting being regarded for this 



