﻿188 TERTIARY COAL FORMATION. July, 



coal," " earth-coal," " conchoidal brown-coal," and 

 " trapezoidal brown coal." Some beds have the 

 external characters of "compact bitumen;" but 

 they generally exhibit in the cross fracture con- 

 centric layers, although from their jet-like compo- 

 sition the nature of the woody fibres cannot be 

 detected by the microscope. Some pieces have a 

 strong resemblance to charcoal in structure, colour, 

 and lustre. Very frequently the coal may be 

 named a " bituminous slate," of which it has many 

 of the lithological characters, but on examination 

 with a lens it is seen to be composed of comminuted 

 woody matter, mixed with clay and small imbedded 

 fragments resembling charred wood. Crystals of 

 selenite occur in this slate, and also minute portions 

 of resin, or perhaps of amber. When this shaly 

 coal is burnt, it leaves light, whitish-coloured ashes. 

 The shape of the stems and branches of the trees is 

 best preserved when they contain siliceous matter 

 or iron-stone ; and in this case, the bark of the 

 tree is often highly bituminised, and falls off from 

 the specimen. 



From the readiness Avith which the coal takes 

 fire spontaneously, the beds are destroyed as they 

 become exposed to the atmosphere ; and the bank 

 is constantly crumbling down, so that it is only 

 when the debris have been washed away by the 

 river, that good sections are exposed. The beds 



