Rev. Dr. Irving — On Dynamic Metamorphism. 303 



I tave maintained is that during the compression-stage you must 

 have molecular friction (or something equivalent thereto), just as, 

 while crushing is going on, you must have molar friction (if the 

 phrase may be allowed) ; and in both cases you must get some of 

 the energy expended manifested kinetically as heat. My real 

 contention was, that the intensity of such heat would be too low for 

 it to be of any practical importance. 



To sum up, I must confess that in the light of what has been 

 adduced in this paper, I see no reason for unsaying or qualifying 

 anything in my previous letter^ (December, 1890), though I am 

 very sorry if any one's susceptibilities have been wounded. It was 

 quite unnecessary for Mr. Harker to offer me any apology; but 

 I am a little surprised that he should not have read what was put 

 into print as a criticism or stricture upon his own most valuable 

 essay, which was printed in the Eeport of the Brit. Assoc, in 1885. 



The one great factor of mineral change at depths is superheated 

 water,^ the existence of which is dependent on pressure. This both 

 Mr. Fisher and Mr. Harker seem to have overlooked. But the 

 experiments of Daubree and others go to show that its action is in 

 many cases in the direction of the resolution of higher and more 

 complex compounds into simpler and denser mineral forms, rather 

 than in the building-up of more complex out of simpler compounds. 



[Since this paper was written I have received from M. Troutschoff 

 of St. Petersburg, a copy of his interesting paper in the " Comptes 

 Kendus," in which he gives an account of his method of preparing 

 crystalline quartz from a dialysed solution of silica in water by 

 prolonged heating in hermetically-sealed tubes ; and in " Nature," 

 vol. xliii. p. 545, the somewhat startling announcement appears 

 of his successful achievement of the synthesis of hornblende, by a 

 similar method, out of the various oxides which generally enter 

 into the composition of that mineral. These were obtained partly 

 as dialysed solutions, partly as fresh-precipitated hydrates, and mixed 

 in right proportions as such. Each tube before sealing up was 

 exhausted by a Sprengel pump, so that probably most of the un- 

 combined water was drawn away as vapour along with the air. 

 The sealed tubes were heated in a specially-constructed sand-bath 

 for three months to a temperature of 550° C. Under these con- 

 ditions the synthesis of hoi'nblende crystals was effected, together 

 with some pyroxene, a zeolite, and a variety of orthoclase ; and it 

 is not difficult to see that here synthesis of the denser minerals was 

 facilitated by the action under great hydrostatic pressure of the 

 superheated water which remained in the tubes, and separated out 

 from the hydrated materials as the digestion proceeded. M. Trout- 

 schoff applied in fact the conditions which may easily be conceived 

 as having obtained generally in the genesis of the heavier minerals 

 of the Archaean schists. See my work (Ibid, pp. 68, footnote, and 

 91, 95, 96).] 



1 With one exception, -whicli no one has noticed. When I said ' shear-planes ' 

 I should have said ' shear- and thrust-planes.' 



- Compare pp. 10-15 of my " Chem. and Phys. Studies in the Metamorphism 

 of Rocks'' ; also the quotation from Pfaff in App. ii. Note K. The work of Pfaff 

 is not as well known as it should be by English geologists. 



