370 I'lHNEST WAKKKN. 



PhvsicBl chflraptpr Fossil wood. Port Impure jet. Port 



i-ny-sicfticnaractei. Durnford. Mus. No. 232. Durnford. Mus. No.2 



Specific gravity . . i ]-i3 1'25 



Hardness (mineral 

 scale) . . . . I 325 275 



From this table it can be seen that the amount of water 

 held hygroscopically is considerable, being 4'61 per cent, of 

 the total weight in the case of the fossil wood, and 12"88 per 

 cent, in that of the jet. Small pieces of the substances were 

 pounded and carefully weighed, and then heated to 105° C. 

 for two hours. On being weighed again the difference gave 

 the amount of water expelled. After the lapse of twenty- 

 four hours of dry hot weather the powder in both cases had 

 absorbed water from the air, and was of exactly the same 

 Aveight as before heating. 



The large amount of silica (7302 per cent.) contained in 

 this lignite, which has a quite typical appearance, was 

 unexpected. With crossed prisms the section is seen to 

 be for most part isotropic. The silica is clearly in the form of 

 amorphous opal, which must penetrate the actual substance 

 of the tracheide and vessel-walls, since from the comparatively 

 low specific gravity and porous nature of the specimen it is 

 quite obvious that a considerable portion of the cavities of the 

 elements remains empty. 



The analysis shows that the Avood has undergone a far 

 more profound change than Avould have been anticipated 

 from the appearance under the microscope. 



As in the case of the much older fossil coniferous woods in 

 the Coal Measures and higher series much differential 

 movement of the woody tissues due to pressure and strains 

 can be observed in the majority of the specimens (text-fig. 2, 

 D.). We have already seen that the annual rings are not very 

 clearly marked in this wood, and there are consequently no 

 very definite lines of weakness along which compression bands 

 could be formed as in the conifer. The whole mass of the 



