68 JET. 



part of a central core of sandstone showing in places distinct 

 indications of concentric rings traversed here and there by crystalline 

 lines which mark the position of medullary rays. In this case the 

 enter portion of the hlock of wood has been converted into jet, 

 while the inner portion decayed and was replaced by some sedi- 

 mentary material which has partially retained the form of the 

 original wood-structure. The replacement by the sedimentaiy 

 material was not accompanied by the compression which 

 characterised the conversion of the wood into jet, hence the 

 greater breadth of specimens such as that shown in Text-fig. 5, 

 as compared with those which consist wholly of jet. 



The Whitby Museum contains some specimens of jet in which 

 Belemnites are partially embedded, clearly pointing to a soft 

 condition of the substance in which the pointed end of a belemnite 

 became firmly enclosed. There is no reason for supposing that in 

 this case the jet was not formed from wood ; it may indeed have 

 been produced from animal matter, but on the other hand it is not 

 improbable that the altered woody tissue may have been in a soft 

 condition and in a state in which a hard substance may easily have- 

 sunk into it. 



DESCRIPTION OF SPECIMENS. 



(All the examples are from the Upper Lias of Whitby, and from 

 the Bryson Coll.) 



51,601. PI. VIII. Fig. 1. 



This section has been cut from a specimen consisting in part of 

 normal jet of a deep sherry colour and exhibiting the characteristic- 

 structure like that shown in Figs. 5 and 7. A portion of the 

 specimen consists of fairly well preserved sUicified coniferous wood. 

 The importance of this section is the transition shown between the 

 wood and the jet; at the junction between the two, as seen in the- 

 photograph, the conversion of the wood into jet is seen to take- 

 place first between the radially disposed tracheids. Cf. Text- 

 figs. 6 and 7 and Fig. 2, PI. VIII. This section and that 

 numbered 51,451 were probably cut from the same specimen. 



51,639. PI. VIII. Fig. 2. 



A piece of sUicified wood in transverse section showing very faint 

 annual rings and suggesting an Araucarian species. Like the slice- 



