67 



seen in Text-fig. 7 formed along the radial walls of the tracheids ; 

 the rest of the section presents the same spotted appearance as that 

 which characterises the yellow matrix of pure jet. The cavities 

 of the tracheids become filled with an orange-coloured substance, 

 and the outlines of their walls may be faintly traced in a micro- 

 photograph of a section of ordinary jet. 



The contorted appearance of the dark lines shown in a transverse 

 section of jet points to contraction of the wood during its con- 

 version into jet. I have no hesitation, then, in describing the dark 

 lines seen in Figs. 5 and 7, PI. VIII. as having been formed by the 

 production of a deep brown substance along the course of the radial 

 walls of the tracheids, their wavy or zigzag form being due to 



Fig. 5. — Jet partially enclosing a core of stone consisting of an imperfectly 

 preserved cast of a piece of wood. ("Whitby Museum.) 



contraction. The mottled appearance of the lighter - coloured 

 matrix is due to the persistence of faint traces or ' ghosts ' of the 

 membranes of wood- elements. The flattened form of the large 

 blocks of jet met with in the jet-rock is no doubt very largely 

 due to the compression of the wood in the process of its alteration 

 into jet. It has been pointed out by more than one writer that 

 specimens are not uncommon in which a thin layer of jet forms 

 a crust over a core of stone. In specimens of this kind the thickneas 

 is considerably greater than in pieces of ordinary jet. 



Text-fig. 5 represents a diagrammatic sketch of an end-view 

 of a specimen in the Whitby Museum, which consists for the most 



