372 . . ERNEST WARREN. 



These changes probably took place undei- shallow boggy con- 

 ditions with plenty of free oxygen from the air, so that a large 

 part of the oxygen of the original wood remained. In some 

 places the original lignified walls of the wood fibres had so 

 far been altered that the yellowish-i-ed substance had ex- 

 truded out of the cavity of the wood fibre to form flake-like 

 pieces. 



At this stage in the transformation the condition became 

 fixed by the infiltration of silica, and a fossil of the slightly 

 altered wood resulted. By reference to the analysis given 

 on page 369 it will be seen that 76'57 per cent, of silica 

 occurs. Of the 50 per cent, of carbon in ordinary wood 

 only 9"53 per cent, remains. Silicification of some of the 

 Avood fibres is much more complete than in the others, 

 and such fibres (figs. 10 and 11, d.f.) with crossed prisms 

 stand out conspicuously Avitli a central core of the yellow 

 material. 



In fig. 12 the transformation has proceeded further, and the 

 cavities of the pitted vessels have mostly become filled with 

 the yellow substance, also the cells of the medullary rays are 

 much more completely filled. 



In fig. 13 the outlines of many of the medullary cells have 

 disappeared, and the yellow substance has extruded so as to 

 form in some cases continuous sheets appearing as thick lines 

 in the cross-section. The bituminous material in the medullary 

 rays gradually becomes dispersed through the general matrix 

 (m.) which is itself being changed into a similar substance 

 (fig. 14). Some especially silicified wood fibres {d.f.) can be 

 observed scattered through the matrix. 



The final result of these changes is seen in fig. 15, a in 

 transverse section. Traces of the large medullary rays can 

 be seen, which are nmch contorted owing to the great pres- 

 sure that the material has experienced. The general matrix 

 appears more or less homogeneous with a faintly granular or 

 reticular appeai'ance. The substance exhibits a marked ver- 

 tical sti'iation, which is readily seen in longitudinal section 

 (PL XXVII, fig. 23), and the reticular appearance in trans- 



