■ Reports & Proceedings — Cornish Inst. Engineers. 523 



been practically homogeneous and containing about GO per cent 

 of combined silica. All water was then in the atmosphere. As 

 temperature fell water and oxygen were absorbed, crust-formation, 

 foundering, and resorption went on for a long period, producing a 

 flat temj)erature-gradient in the liquid. When the crust became 

 jDcrmanent, granite developed, and below it basaltic magma long 

 remained liquid. At this stage the isostatic balance was adjusted. 

 Ore-minerals in large quantity were given off at the surface of the 

 granite, were denuded, and dispersed in sediments and solution. 

 This, with subsequent absorption by basaltic magma, is assumed to 

 have been instrumental in causing the present erratic distribution of 

 primary ores. 



Magmatic differentiation is regarded as having been caused by 

 the agency of silicic acid — silicon combined with hydroxyl — which 

 extracts potash alumino-silicate, producing a solution immiscible 

 with and lighter than a melt of basic felspars and femic minerals. 



Evidence regarding the existence of silicic acid in magmatic 

 liquids and elsewhere in nature is recited. 



The ultimate result of the action of water on rock magmas is that 

 silicates are completely removed and a residue of ore-minerals — 

 magnetite, etc.— left. 



Vein iissuring is held to have been brought about in and above 

 batholiths by the expansive force due to the increase in solid specific 

 volume of various elements. The effect of this force appears in waves 

 as successive series of fissures. These developed very rapidly, were 

 instantly filled with magmatic mother-liquor, and quickly sealed 

 by the deposit of solids therefrom. It is argued that primary ore- 

 minerals passing up from magmas to veins do so in silicic acid 

 solution ; that their deposition is determined both by loss of heat 

 and reduction of pressure ; and that there are definite but narrow 

 limits of temperature between which each ore-mineral develops. 



Reasons are given why the pneumatolytic origin of tin and 

 tungsten ores should be rejected. 



Cornish Institute of Engineers. 



September 16, 1920. 



'■ On the Characters of some Cornish Veinstones." By the 

 President, Mr. E. H. Davison. 



The paper gives the results of the examination of a number of 

 Cprnish lodes. Those not at present being worked were represented 

 by isolated specimens from known points in the lode, while the lodes 

 now working were systematically sampled down the dip. 



The methods employed included microscopic and microchemical 

 examination as well as in some cases mechanical analysis. It was 

 considered that three types could be recognized which were related 

 to the surface of the nearest granite mass. 



