On the Genesis of Some Scottish Minerals. 219 



might be gradually converted into that aggregate of crystalline 

 felspar in hands alternating loith those of other rock-forming 

 minerals, to which the term Gneiss is correctly applied. 



The separate stages in the conversion of a deformed rock 

 of sedimentary origin into the mylonitic, granulitic, augenitic, 

 pegmatitic, and gneissose grades of structure, on the one 

 hand, or into a truly eruptive rock, on the other, are 

 by no means well defined. This fact is easily explained on 

 the supposition that these various grades have arisen in the 

 manner suggested ; but it is certainly difficult to explain 

 the fact by any other hypothesis that has yet been put 

 forward. 



Where the rocks affected contain undecomposed felspar — 

 as in the case of original eruptive rocks, on the one hand, 

 and the arkoses derived from them under desert conditions 

 on the other — there is little or no difficulty in accounting for 

 the facts. 



It may be as well now to summarise the leading ideas 

 put forward in the latter part of this communication, as 

 it is these which are likely to meet with least favour at 

 present : — 



Alkalies are released by weathering from the surface of 

 the eruptive rocks on the land, are carried in solution by 

 rivers to the sea, thence they are transferred by osmosis into 

 the sediments below the sea-floor, and especially so within 

 the marginal zones of the sea within which terrestrial 

 activity, and, therefore, the generation of heat by dynamic 

 causes, attains its maximum. Aided by the heat thus 

 generated, and by occlusion, the alkaline waters, in making 

 their way into the rocks, slowly give rise to new compounds, 

 and in the bathymetrical zones where terrestrial movements 

 are most active, the materials tend to pass into the fluid 

 condition, which stage they reach directly the necessary 

 relief of pressure takes place (see Geol. Mag., Jan. 1894, p. 25). 



With any sudden relief of pressure in the higher zones, 

 it is probable that the occluded water suddenly flashes into 

 steam, and gives rise to the subterranean concussion to which 



