240 MR. J. B. HANNAY ON SILICEOUS FOSSILIZATION. 



Fig. 6 is a small splinter^ the centre t)f which was in a 

 higher state of hydration than the exterior^ so that, when 

 treated with hydrofluoric acid, it left a hollow interior. 

 The portion left shows the layers also. 



I noticed, when rendering some of the opalescent rods 

 transparent, that they did not all show the same equally 

 distributed transparency ; but when viewed by transmitted 

 light, dark rounded patches of opaque matter appeared 

 irregularly throughout the transparent portions, as shown 

 by fig. 3. When viewed by reflected light, they showed 

 themselves as white patches upon the opalescent ground, 

 as in fig. 10. It was found that by heating these rods in 

 dilute caustic-soda solution (about 2 per cent.) in sealed 

 tubes to 130° C, the transparent portion could be dissolved 

 away, leaving rounded nodules held together by a portion 

 of the opalescent substance, which, viewed by reflected 

 light, had the appearance given in fig. 1 1 . 



Dr. Young has figured in his paper rods which are made 

 np of nodules, and which present very much the same ap- 

 pearance as shown in the above figure if the whole of the 

 opalescent portion were dissolved away ; and it would 

 appear as though they had been formed in somewhat the 

 same way — that is, by the solution of the more highly hy- 

 drated portions. The question is, how are the rounded 

 nodules formed in the rod ? One of the largest I could 

 find was carefully split, and its structure examined ; and it 

 was seen, when viewed under the microscope by reflected 

 light, to have the appearance presented in fig. 14 ; that is, 

 it had the character of a radiating nodule of crystalline 

 silica. 



Now it appears from the above evidence that in this 

 case of siliceous fossilization the process goes on in some- 

 thing like the following way. The clear transparent rods 

 first lose a portion of their water, becoming transformed 



