SOUTH OF REDRUTH AND CAMBORNE, ETC. 647 



II. Probable Mode of Origin of the Great Flat Lode. 



We have now to consider the mode of origin of these deposits 

 of tin. 



Their distinctive features are : — 



1. The invariable presence of a small leader, generally only a 

 few inches wide, apparently occupying the space due to the shift- 

 ing of the two sides of a comparatively nattish fissure, and filled 

 partly mechanically, partly chemically. 



2. A lode or mass of stanniferous schorl-rock, containing 1 to 

 3 per cent, of cassiterite, from 4 to 15 feet in width, either above, 

 below, or on both sides of the leader. The tin is distributed in 

 little grains in the rock or in strings or minute veins. I believe 

 that the fact of the lodes themselves consisting of schorl-rock is 

 a point which has never before been made known. 



3. Schorl-rock, poor in tin ore (locally called capel, greyback, 

 black granite), separating the lode from the granite, and schorl- 

 rock with its constituent minerals arranged in layers, also called 

 capel, separating the lode from the killas. 



4. Gradual passage of the schorl-rock (cajpel) into lode on one side 

 and into granite or killas on the other ; in other words, absence of 

 any wall between the capel and the lode, or between the capel and 

 the enclosing rocks. 



All these facts point, in my mind, to the idea that the lode and 

 capel are merely altered rocks, the fissure now occupied by the 

 leader having served to bring up vapours or solutions capable of en- 

 tirely changing the rocks on both sides of it. It is very difficult, 

 I admit, to conceive that this compact blue stanniferous schorl-rock 

 could once have been granite ; but I may further mention that both 

 at South Condurrow and Wheal Grenville I have found cavities as 

 big as a pea which seem to agree in form with crystals of orthoclase ; 

 and the microscopical examination already referred to affords con- 

 firmatory evidence on this point. 



III. Alteration of Granite in other Districts. 



That granite can be altered into schorl-rock is a fact that can 

 scarcely be questioned. Every china-clay pit affords us evidence of 

 the fact. You constantly find little veins of quartz, or quartz and tin- 

 ore, bounded on each side by granular schorl-rock, which is followed 

 by granite more or less decomposed. I have specimens in my pos- 

 session of a passage-rock showing pseudomorphs of gilbertite after 

 orthoclase enclosed in schorl and quartz. The sharp walls of the 

 quartz-veins on the sides of the original fissures and the absence of 

 such walls between the granite and the schorl-rock is what would be 

 expected if the altering solutions gradually soaked in from the sides. 

 Sometimes the schorl-rock itself bears plain marks of its origin, as it 

 contains numerous pseudomorphs of quartz after orthoclase. This 

 is nowhere better seen than in some rocks occurring on the granite 



