66 



reveal no definite structure in tie homogeneous substance, or, more 

 frequently, the matrix is seen to be traversed by a number of dark- 

 brown undulating, zigzag, or roughly parallel lines (PI. VIII. 

 Figs. 5, 7) ; while in other sections the matrix exhibits a series of 

 faint vertical Unes crossed here and there at right angles by short 

 dark lines (PI. VIII. Kg. 3). A transverse section of jet usually 

 appears as represented in Figs. 5 and 7, PI. VIII. ; the contorted 

 dark lines reveal no indication of their origin or meaning, and the 

 rich yellow or orange matrix shows nothing more than an indistinct 

 spotted appearance too faint to afford a clue as to its origin. 



It is interesting to find that a miorophotograph of a section such 

 as I have described reveals more distinct evidence of structure in 

 the matrix, and brings to light more definite though indistinct 

 traces of a fine net-like texture suggestive of compressed plant- 

 tissue, than the section itself. A section cut parallel to the edge 

 of a compressed mass of jet — that is, a tangential section — exhibits 

 a number of slightly irregular vertical lines crossed at intervals 

 by series of short dark patches (PI. VIII. Fig. 3). This structure 

 is obviously a faint representation of that of a coniferous stem, the 

 vertical lines being the walls of the tracheids, while the darker 

 patches mark the position of the medullary-ray cells. Fig. 4, 

 Plate VIII. represents a specimen one edge of which is composed of 

 pure jet having the structure seen in Figs. 5 and 7 ; the rest of the 

 slice consists in part of mineral material, showing little or no 

 trace of woody structure, with occasional patches of fairly well 

 preserved petrified wood cut across transversely. 



Figs. 1 and 2 represent portions of sections which are in part 

 composed of jet and in part of siUcified coniferous wood; these 

 preparations show the extension of the dark and apparently 

 structureless jet into the tissues of the wood, and in the wood 

 itself one sees that the dark lines follow the course of the radial 

 walls of the tracheids. Similarly, in Fig. 6 we have a piece of 

 coniferous wood in transverse section invaded by dark lines which 

 clearly follow the course of the rows of tracheids and extend along 

 their radial walls. Text-fig. 6 shows a small piece of wood in 

 transverse section of which the tracheids are more or less ragged 

 and disorganised, the tangential walls have partially broken down, 

 while the radial walls are being replaced by a brown substance 

 which forms conspicuous dark bands («). Identical dark bands are 



