GEOLOGY. 345 



pyrites and lignite in the form of scattered trunks of trees. Where it is, 

 from drainage, subjected to subaerial decay, tlie pyrites is oxidized, 

 and the silicates are decomposed with liberation of bases and much 

 silica, which is deposited in a crystalline form in the fissures and cavi- 

 ties of the gravel, and also produces a veritable petrifaction of the lig- 

 nite, by a process which is marked by three stages. The first of these 

 is the filling up of the pores in the wood, the organic tissues remain- 

 ing ; the second, the slow removal of these by oxidation, and the third, 

 a filling up the vacant spaces thus left. After the second stage, the 

 silicious casts of coniferous wood are often separable in the form of 

 fibers which have been mistaken for asbestos. A similar condition of 

 things has been observed by Kerr in the auriferous gravels of Forth 

 Carolina, where also the decay of silicates and the silicification of fossil 

 wood is going on. 



The oxide of tin, cassiterite, which, like quartz, occurs crystallized in 

 ancient granitic veinstones, appears like it also to have been deposited 

 from solutions in recent times. Collins, in the Transactions of the Royal 

 Geological Society of Cornwall, has described his examinations of deer's 

 horns from the tin-bearing gravels of the region, which are impregnated 

 with cassiterite, and even contain visible crystals of the mineral. Some 

 of these horns are so rich in tin-ore as to be sought by the smelter. A 

 specimen examined by Collins of the horn of the red deer ( Cervus ela- 

 phiis) contained 2.6 per cent, of oxide of tin and 1.6 per cent, of pyrites, 

 both of which were seen by the microscope to be inclosed in the cells 

 of the horn. The process of stannification is thus, like that of silicifi- 

 cation, one in progress in modern times, though under conditions as yet 

 unknown to us. 



