308 ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 



Duporth, Clicker Tor, and Polyfant, and the pseudo-basalts of 

 Brent Tor were probably among the latest of these pre-granitic 

 eruptives. The latest of all were, perhaps, the mica-traps 

 which have been traced as more or less continuous dykes, running 

 in a meridional direction from St. Keverne, some miles south of 

 the Helford river, up the Truro river, past Liskes and CargoU to 

 the north side of the Valley of the Gannell near Newquay. This 

 series* is pretty certainly much older than the oldest of the 

 felsitic or granitic elvans, although actual intersections of the 

 mica-traps by such elvans have not been seen except, perhaps, at 

 Treliske. 



4. The principal cleavages and jointings of the rocks, as 

 we now find them, were also in existence ; but the contact 

 metamorphism which is now so obvious was wanting, and there 

 were few, if any, metalliferous deposits other than certain bands 

 of magnetite in the basic eruptives, and, perhaps, the cupriferous 

 and garnetiferous belt at Belstone, near Okehampton. Fossils 

 were much more numerous, and much better preserved, but 

 there were probably many beds entirely unfossiliferous. 



5. The granitic intrusions only slightly affected the then 

 existing strike and dip of the stratified rocks. But the strains 

 occasioned by these elevatory movements could not fail to pro- 

 duce openings in the overlying strata, thus establishing channels 

 through which stanniferous and other mineral solutions 

 could rise from the depths, so as to impregnate and 

 metamorphose the rocks for many hundreds of yards, thus pro- 

 ducing what has been called the " granitic penumbra." The 

 mineral springs thus established probably contained a great 

 variety of chemical substances, including especially the char- 

 acteristic constituents of cassiterite, schorl, and various metallic 

 sulphides. t 



*Described by the author, Journ, Roy. Inst. Corn., Vol. VIII, p. 190. 1884. 



f It is pretty certain that soluble compounds of fluorine and boron were more 

 abundant in these early solutions than the tin itself, and these acting upon the iron 

 and silica already present in the stratified rocks, would be sufficient to produce 

 all til e tourmaline now seen in the " tourmaline schists" and "schorl rock" 

 which are so characteristic of the junctions of granite and " killas," as well as in 

 that found in the early tin-lodes and in the tin strings of the stock-works. 



