JET. 65 



other. He considers that it was not formed from plants which 

 grew where the jet is found, as it does not occur in regular seams, 

 but in isolated blocks. It is more probable, he adds, that jet was 

 produced from trees which had been floated into their present 

 position, buried in Lias mud, and subsequently converted into 

 bitumen, " which became diffused through the neighbouring shale 

 or occupied cavities in them, where it is now found as jet, often 

 occurring as pseudomorphs of organic remains." ' 



This brief historical review suffices to show that the balance of 

 opinion is in favour of a vegetable origin for jet. Several writers, 

 however, prefer to regard the masses of flattened jet, having the 

 form and texture of wood, as pieces of stems or branches 

 impregnated with a fluid bitumen, rather than as pieces of wood 

 which have been directly converted into jet. With the exception 

 of Witham, none of the authors who have discussed the origin of 

 jet appear to have made a microscopical investigation of its 

 structure. 



The result of an examination of several sections of Whitby jet 

 in the British Museum collection leads me to express the opinion * 

 that jet has been formed by the alteration of wood : the masses 

 of wood with the form and structure of jet that were formerly 

 extensively worked as the source of that material, probably 

 represent portions of trees the tissues of which underwent certain 

 chemical changes resulting in the partial or complete obliteration 

 of the vegetable structure and in the substitution of jet for wood. 



In all probability Whitby jet has been produced in large 

 measure by the alteration of wood of the Araucarian type. 

 Undoubted Arauoarioxylon wood occurs in the Lias rocks in 

 association with the jet (Plate YI.), and the structure of the tissues 

 met with in specimens Hke those shown in PI. VIII. Figs. 1, 2, 4, 

 and 6, appears to be of the same type. It is interesting to find 

 that Knowlton speaks of jet found in the Kiohmond Basin, Virginia, 

 as having been formed from Arauomrioxyhn wood.^ 



A section cut from a good sample of hard Whitby jet is of a rich 

 sherry or golden yellow colour. Microscopical examination may 



' Fox-Strangways (92), p. 455. 



^ Brieiy stated in a note published in 1901 ; see Seward (01), p. 856. 



' Knowlton, in Staler & Woodworth (97), p. 517. 



