286 ON THE TRANSMUTATION OF ROCKS, &c , 



Lignite, one of the lowest forms of combustible, sometimes 

 becomes prismatised, at other times it is only found in the form 

 of charcoal which occurs in all coal seams more or less ; just as 

 it is found in lava from Madeira, where wood of a recent tertiary 

 epoch is entangled. 



At Andernach, and in other places visited by me on the Rhine, 

 I saw trunks of trees and leaves only partially carbonised in a 

 hydrated volcanic tuff. In lava near Neuwied, carbonised trees 

 in an upright position just as they grew, may be seen. 



In a volcanic tuff, or ash bed, at Ko3nigswinter, under Drach- 

 enfels, I found stems of trees converted into semiopal, just such as 

 occurs in Tasmania. Here are specimens for comparison. 



At Herculaneum, a beam has been found in the volcanic tuff 

 which covered that city, preserving its woody character; and, 

 according to Delesse, Pelouze found in it 50 per cent, of carbon. 



With respect to the prismatising of coal, we must not be led 

 into errors from the fact that similar effects occur from artificial 

 heat in the chemise or lining of a furnace, for simple desiccation 

 will prismatise some combustibles ; and it is stated, on authority 

 of careful experiments, that such prismatised combustibles are 

 never at a red heat. In fact, prismatisation is a minor transmu- 

 tation than that of coking. 



This is well illustrated by the oil-bearing Cannel of Mount 

 York. Though full of oil, it has undergone transmutation in a 

 perceptible degree ; its laminse are obliterated, the strata lines 

 are merely stripes, whilst the blocks into which it is separable are 

 distinctly, though rudely, prismatic. It is difficult to separate it 

 in planes which present unbroken its distinctive impressions of 

 Glossopteris, a fossil plant which marks its epoch. A further 

 proof of transmutation is exhibited over it, where a silicate of 

 alumina occurs full of bright crystals of pyrites, which mineral 

 has thus resulted from the removal of the sulphur from the 

 cannel. 



One proof that water must be present in these changes, is the 

 occurrence of zeolitic minerals (which are not common near 

 granitic contacts where the water is less), and also by hydroxydes 

 of iron and alumina. This element has been already mentioned 

 in relation to granite ; but heat must also be occasionally widely 



