69 



represented in Fig. 1, this also shows the gradual encroachment 

 of the jet along the rows of tracheids. In sections' such as those 

 shown in Figs. 1 and 2 it is not difficult to trace the dark lines and 

 patches, clearly shown in the photographs as ocourring'in the wood, 

 into the wavy and contorted bands which are seen in the portions 

 ■of the specimens already converted into jet. 



51,595. PI. VIII. Kg. 3. 



An obliquely tangential section of pure jet of a rich sherry 

 colour. The medullary rays are shown as superposed rows of dark 

 patches and in some places as more radially extended lines. Both 

 tracheids and medullary -ray cells are represented by faint lines or 

 darker patches, and afford a blurred and obscure reproduction of the 

 actual wood. 



51,493. PI. YIII. Fig. 4. 



The section, slightly reduced in the photograph, is 4-5 cm. in 

 length : it consists of bands and patches of ordinary jet inter- 

 spersed with badly preserved silicified wood seen in transverse 

 section. The jet presents the same appearance as that represented 

 in Figs. 5 and 7. In many parts of the section the passage from 

 wood to jet is clearly shown. 



51,623. PI. VIII. Figs. 5 and 7. 



Sections cut from a piece of pure jet, which has a rich brown 

 colour in transmitted light. The lighter - coloured matrix is 

 traversed by dark opaque bands exhibiting small zigzag contortions 

 (Fig. 7) and large folds. The matrix between the bands resolves 

 itself, under a high magnifying power, into a network of more or 

 less rectangular spaces representing tracheids, the cavities of which 

 are bounded by very fine liues. Under a lower power the matrix 

 appears to be without structure and homogeneous. The wood from 

 which the jet has been formed is cut across transversely; the dark 

 bands probably mark the position of the medullary rays, and their 

 junction can be readily traced in such specimens as those repre- 

 sented in Figs. 1 and 2 ; the mottled appearance of the matrix, as 

 shown in the photographs (Figs. 5 and 7), is simply the faint 

 remnant of the wood. 



51,448. PI. VIII. Fig. 6. 



A small piece of a specimen similar to that represented in Fig. 4, 

 consisting of a dark patch of jet and portions of silicified wood in 



