64 



a crust on the outside of a log of pyritised or silicifled wood. 

 Wittam speaks of jet as possessing a "ligneous structure," and 

 as exhibiting "concentric rings of fibrous tissue." He was the- 

 first to give a figure of a section of jet showing indications of 

 a woody structure.' 



An interesting paper on Whitby jet and its manufacture, by 

 Mr. J. A. Bower, was published in the Jowrnal of the Society of 

 Arts in 1873." This author deals at length with the history of the 

 working of jet, and briefly discusses its probable origin. He notes 

 that most of the jet-workers regard it as having a ligneous origin. 

 Bower does not consider that vegetable matter is essential for 

 its production, and in support of this opinion he mentions the 

 occurrence of bones and fish-scales which have been converted into 

 jet. Jet, he says, has the appearance of a substance distilled from 

 the rock, which has in some cases impregnated vegetable matter 

 and animal substances, or has simply consolidated in fissures. On 

 the whole he considers that the evidence supports the view that 

 jet has been formed as a distillate from the jet-rock. 



Messrs. Tate & Blake have dealt with the origin of jet in their 

 book on the Yorkshire Lias : they point out that the beds in which 

 it occurs are highly charged with bitumen, and they consider that 

 the jet owes its origin to " the segregation of the bitumen in the 

 intervals of the shales, which, allowing to a certain extent the access 

 of air, has hardened into jet, a process which may undoubtedly be 

 now going on." They go on to say, " There seems to be no reason 

 whatever for connecting it with wood, beyond its having a remotely 

 similar composition, though, of course, we have thrown no light on 

 the cause of the presence of the bitumen itself." ^ 



Jet is described by Messrs. Fox-Strangways & Barrow, in the 

 Geological Survey Memoir on the Coast between "Whitby and 

 Scarborough, as water-logged Coniferous wood "from which in 

 a few rare cases all trace of structure has been removed." * In 

 a more recent work, Pox-Strangways points out that it is very 

 certain that jet is derived from vegetable matter in some form or 



' "Witham (33), pp. 10, 50, pi. xi. fig. 3. 



2 Bower (73). 



3 Tate & Blake (76), p. 179. 



* Fox-Strangways & Barrow (82), p. 21. 



