366 ORIGIlSr AND DEVELOPMENT OF ORE-DEPOSITS. 



the beds iu question. We can therefore easily understand why 

 these tin-bearing rocks have yielded no fossils, since no higher 

 forms of life at any rate could exist in a sea charged with 

 highly poisonous soluble compounds of tin-fluorides, fluo- 

 chlorides, fluo-borates, and fluo-silicates.*' 



As for the tin stoclvworks in killas which are associated with 

 definite lodes, such as Great Wheal Fortune and Pednandrea, 

 Von Ootta's hypothesis as applied by H. C. Salmon to the great 

 "underground stockwork " at the latter mine in 1862 is as 

 follows: "I consider this great deposit as eminently characteristic 

 of a class of stanniferous formations common enough in Cornwall, 



but which are usually classed as lodes The real fact is that 



there was a fissure, but often only a very small one, fromi which 

 a metamorphosing and replacing action appears to have 

 emanated, extending to a greater or less distance, metamorphos- 

 ing the neighbouring rock into a capel and impregnating it by 

 replacement with oxide of tin. When this is confined to a 

 moderate width, and where the tin does not extend away in veins 

 at right angles to the lode, the miner classes it all as a " lode," 

 and properly so. As to "walls" which some appear to consider 

 the criterion of a true lode, they may in these highly dislocated 

 districts be frequently met with ad infinitum ; and in such deposits 

 as those referred to it is not unusual to find that half-a-dozen 

 " walls " have been adopted in succession as the true wall of the 



lode and abandoned I do not of course mean to imply by 



this that there are not many lodes wholly confined within the 

 walls of an original fissure — lodes in the popular geological sense. 

 I only wish to point out that there are many lodes of a different 

 character, and this Pednandrea deposit is characteristic and 

 worthy of study as forming a link between lodes of this class 

 and those still more irregular deposits called carbonas in the 

 extreme western districts of Cornwall."! 



*0n this subject the following' suggestion was made by my son H. F. 

 Collins and myself in the year 1884. " The waters were so strongly impregnated 

 with chemical solutions — from mineral springs preceding the granite irruptions — 

 th&t nothing could live. If this were the case the sediments would also be highly 

 charged with chemical substances, and the subsequent segregation of these 

 substances into fissures formed at a later date has given us the lodes of Cornwall." 

 Journ. R.I.C., Vol. viii, part 2, p. 166. 



■f Mining and Smelting Magazine, Sept., 1862, pp. 143-4. 



