Major-Gen. McMahon—Tke " Culm " at Bade. 223 



which he states are "quite distinct from the clastic grains of that 

 mineral." I noted in my paper that the specimens of the Bude 

 rocks examined by me contain fragments of schorl (tourmaline) 

 and needles imbedded in quartz grains that may be referred to this 

 mineral. Mr. Hutchings does not explain in what respect the 

 crystals on which he relies are "quite distinct" from the "clastic 

 grains," or whether these crystals are imbedded in or attached to 

 quartz grains or not. If he means that they present idiomorphic 

 shapes, and are not obviously fragments, that fact does not prove that 

 they were formed after the beds that contain them were deposited. 

 I have in my collection minerals showing very perfect crystallo- 

 graphic shapes collected from river sands. Mr. Dick showed the 

 presence, not only of " sharply-edged prisms " of rutile, but of 

 " perfect crystals " of tourmaline in the Bagshot beds at Hampstead. 1 

 I was at pains to show in my paper that the material of which the 

 Bade Culm-measures is composed did not travel far ; that the cui*- 

 rent which deposited it was very sluggish ; and that the granules of 

 which the Culm-measures are built up are not water-worn. 



With reference to Mr. Hutchings' remarks on " Hallock's experi- 

 ments and conclusions, it seems sufficient to observe that so good 

 an authority as the Rev. 0. Fisher thought Mr. Hallock's experi- 

 ments and conclusions sufficiently valuable to quote in the second 

 edition of his " Physics of the Earth's Crust." 2 



I turn now to Mr. Harker's letter. I think the readers of the 

 Geological Magazine are to be congratulated on this interesting 

 and able contribution to the discussion of dynamic metamorphism. 

 Nothing could be more lucid than Mr. Harker's short exposition of 

 the way strains in rock-masses resolve themselves into voluminal 

 compression and shearing. I see no room to differ from Mr. Harker's 

 statement of the principles that govern strains; and in the following 

 remarks my object is to make my own position clear, rather than to 

 criticize his letter. 



In my paper on the Bude rocks I expressly restricted my con- 

 clusions to the results of " pressure alone." Mr. Harker's remarks, 

 on the other hand, refer to the effects of " (i) uniform voluminal 

 compression, and (ii) certain shears ; " that is to say, to the two 

 principal mechanical sources of heat. 



To quote from the Rev. O. Fisher's Physics of the Earth's Crust 

 (second edition, p. 2) : " Pressure by itself cannot develope heat. 

 That takes place only when some kind of motion, which however 

 slow may be termed " visible," is arrested and transformed into that 

 invisible motion of the ultimate molecules of a body which con- 

 stitutes heat." On the other hand, the fact that voluminal compression 

 and friction (shearing) are mechanical sources of heat belongs to the 

 elements of physics. 



If " pressure alone " cannot develope that form of molecular 

 energy known as heat, it cannot develope those other forms of energy 

 into which heat is convertible. 



1 Nature, vol. xxxvi. p. 91, and plate 44, fig-. 3, British Petrography. 



2 Page 172, footnote. 



