ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 147 



subsequent oxidation change of the chloritic portion of the mass.* 

 The poorer parts of the lode are very similar, except that they 

 are very much harder and more siliceous, and with a tendency to 

 a black colour. 



The country rook changes noticeably with the lode matter. 

 The killas is fairly soft near the surface ; it generally becomes 

 harder as it approaches the granite, but is always soft near the 

 rich parts of the lode ; above it is often yellow or buff in colour, 

 below mostly deep blue. While the granite remained very hard 

 the lode was not productive either for tin or copper — it is now 

 in general fairly soft and moist, the felspar much kaolinized and 

 often accompanied with pyrites and red oxide of iron. 



The curious alternations of killas and granite which this 

 great lode cuts through have been noticed by many writers. 

 In 1882 "a large mass of hard slaty rock was met with in the 

 352 fathom level east of the new eastern shaft. , . .included in 

 the granite 240 fathoms below the point where that rock was 

 first cut into by the workings .... this resembles the ordinary 

 killas of the district, and on comparing thin sections of the two 

 under the microscope their identity becomes at once apparent."! 



The imagination is struck with the figures expressing the 

 extent and the produce of this great lode. But if we look at 

 the facts in another way, as suggested by M. Moissenet, we shall 

 see how insignificant a feature it forms in the earth's crust. 

 Let us suppose a model of the lode made to a scale of one 

 thousandth the real size — it could be easily made from a sheet 

 of lead 12-feet long and 3-feet wide. In many places the 

 thickness would have to be reduced to a mere film, but in some 

 it would require to be thickened up to a quarter of an inch or a 

 little more. The sheet might be placed on edge — its length in 

 a direction nearly N.E., S.W., and with a considerable dip to the 

 southward. If now it were bent lengthwise in such a way that 

 the thicker portions were more nearly E.W. and the thinner 

 more N.E., S.W., and also bent in width so that the thicker 

 portions stand more nearly vertical than the thinner, and the 

 lower portions more nearly horizontal than the upper ; it would 

 very fairly represent the relative proportions of the lode and 



*See Cornish Tin-stones and Tin-Capels, pi. iv, figs, 3-4. 

 t Phillips, Ore deposits, pp. 131-2. 



