CANNA. GEOLOGY. 449 



the minute division and intricate positions of the prismatic 

 concretions. To give a lively conception of its appear- 

 ance it may be compared to a peat stack; the prisms 

 resembling in shape and size, as well as in colour, the 

 masses of this combustible after they have been dried 

 for use. 



Bituminous wood occurs in many places among these 

 rocks, being sometimes entangled in the apparently solid 

 trap, at others, accompanying the various substances that 

 enter into the conglomerate. It has much of the charac- 

 ter of oak, and is always bent or distorted ; occasionally 

 it passes into black wood coal. When first taken from 

 its repositories it is flexible and moist, but in a few days 

 becomes hard, and in no long time falls to pieces. In 

 this respect it resembles many rocks which in their native 

 situations contain water, but which lose it after a short 

 time and are incapable afterwards of being restored to 

 their original state. 



On the shores near the low water mark, beneath the 

 columnar rocks now described, is found a bed, or a series 

 of beds of a trap conglomerate. This, although the lowe'^pr 

 substance near the harbour where it is most accessible, is 

 not geologically the lowest rock, columnar trap being 

 found beneath it both here and very conspicuously in 

 the neighbouring and connected island. It must there- 

 fore be considered as alternating with the other kinds of 

 trap ; but these alternations, like those of the several 

 varieties above mentioned, are too irregular and capricious 

 to admit of a detailed description. The nature of the 

 beds requires however to be described. They vary iii 

 structure and general aspect, although composed almost 

 exclusively of fragments of the several kinds of trap. 

 In some cases they consist of an irregular mixture, more 

 or less compact, of angular fragments, of various sizes, 

 resembling at first view a heap of loose rubbish, while 

 in others they are chiefly or entirely formed of that friable 

 aggregate of earthy trap to which the name of tufo seems 



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