326 REGIONAL METAMORPHISM. [1885-86. 



I fear the rock may have been carted to the spot to mend the 

 road. Vessels are constantly arriving in the Thames in ballast, 

 which often comes in usefully for road-mending. It is common 

 to find rocks from China and Japan on the London roads. When 

 you are next at Green Street Green can you make some inquiries 

 about it of roadmakers or others ? , 



An estimate of his paper on " Regional Metamor- 

 phism " is given by an eminent American geologist in 

 the following letter : 



Prof. Joseph le Conte to J. Prestwich. 



BERKELEY, CAL., 18th November 1885. 



MY DEAR SIR, I need not tell you how deeply interested I 

 have been in your paper on metamorphism. I had already read 

 an abstract of it in ' Nature ' for July 3rd, but am very glad to 

 have a fuller copy. I have long believed that crushing is an im- 

 portant source of the heat of metamorphism, and have spoken of 

 it in that connection in my ' Elements of Geology,' under Meta- 

 morphic Rocks and under Volcanos and the source of their heat. 

 But I have never, I believe, given it sufficient prominence, 

 and I am glad that you have now done [so], especially that 

 you have brought forward positive evidence in the case of the 

 St Gothard Tunnel. 



I have also noted with great interest your views of the sources 

 of volcanic water and of volcanic force. I have no doubt that the 

 violent explosions of many volcanos are due to superficial water, 

 but even in the quietest eruptions, as in Hawaiian volcanos, there 

 seems to be considerable water in the lavas. Let me draw your 

 attention without comment to Button's memoir on the Hawaiian 

 Islands, Fourth Annual Report of United States Geological Survey 

 (the page I cannot now refer to), where he gives reason for think- 

 ing that water is intimately incorporated with igneous magmas, 

 not as vapour vesicles but as a sort of hydrate, and does not 

 separate in vesicles until the lava is about to solidify. So that 

 lava after running 40 miles, and therefore after many days' ex- 

 posure to atmospheric pressure only, still solidifies as a light 





