ANNIVERSARY ADDRESS OF THE PRESIDENT. CXX1X 



the present — a conclusion which equally applies to other minerals 

 found in the veins of various rocks, — whilst the many fluid- cavities in 

 the constituent minerals of mica-schist and the rocks associated with 

 it show that the metamorphism to which they have been subjected was 

 due, at least in part, to the action of heated water, and not alone of 

 dry heat and partial fusion — a conclusion which should be com- 

 pared with some of the deductions of M. Delesse. 



The structure of the minerals in erupted lava shows that they were 

 generally deposited from a mass in the state of igneous fusion ; but 

 as in some of the blocks ejected from volcanos, crystals are found 

 which contain water-cavities, in. addition to stone- and glass-cavities, it 

 may be fairly assumed that they were formed under great pressure, 

 when both liquid water and melted minerals were present, and that 

 the minute crystals which the fluid-cavities of these aqueo-igneous 

 minerals generally contain have been deposited on cooling from the 

 highly-heated water which once filled the cavity. The minerals in 

 trap-rocks appear to have been of genuine igneous origin, though 

 they have been subsequently much altered by the action of infiltra- 

 ting water, from which many other minerals have been also deposited. 



The quartz from quartz-veins appears to have been rapidly de- 

 posited from solution in water, the temperature of which must, from 

 the diminished volume of the water by cooling, have been consider- 

 able, about 329° ; at a still higher temperature, mica, limestone, and 

 probably even felspar were deposited, showing, as has been asserted 

 by M. Elie de Beaumont, a passage from quartz-veins even to granite, 

 and that the one therefore can scarcely be separated, as a rock of depo- 

 sition, from the other as a rock of purely igneous fusion. Whilst, 

 indeed, in the quartz of highly quartzose granite, the liquid- cavities 

 are so numerous as to number millions even in a cubic inch, and the 

 water they contain amounts to 1 or 2 per cent, of the volume of the 

 quartz, so that they might be considered the result of an aqueous 

 solution, both the felspar and quartz exhibit stone-cavities exactly 

 similar to those in the crystals of furnace- slags or of erupted lavas, 

 and therefore indicate the conjoint action of igneous fusion. The 

 great conclusion which Mr. Sorby draws from this fact is, that gra- 

 nite is not a simple igneous but rather an aqueo-igneous rock, pro- 

 duced by the combined influence of liquid water and igneous fusion, 

 under similar physical conditions to those existing, far below the 

 surface, at the base or focus of modern volcanos. These deductions 

 of Mr. Sorby are in conformity with the views of Scrope, Scheerer, 

 and Elie de Beaumont ; and he even agrees with them in consider- 

 ing it probable that the difference between erupted trachytic rocks 

 and granite, both of which are included by M. Durocher (in the paper 

 on which I have already commented) in the same class, as proceeding 

 from the same magma, is due to the presence or absence of the water. 



In whatever light we regard the paper of Mr. Sorby, whether as 

 founding upon microscopical observations a new explanation of the 

 conditions under which rocks have been formed, or as proving from 

 them the intimate association of water with the constituents of mi- 

 nerals, independently of the simple water of crystallization, it cannot 



