CAXNA. GEOLOGY. 453 



fctitution of the first variety" it will be found, that in- 

 dependently of the rolled masses, it occasionally contains 

 fragments of columnar basalt and of organic materials; 

 that is, of trees bituminized and flattened as if by the 

 pressure of a superincumbent weight; substances ana- 

 logous in their chemical characters and general forms 

 to jet and surturbrand. The imbedded pebbles will be 

 seen to consist of the various modifications of trap which 

 are often known to occur in the same set of rocks ; 

 of basalt, of common greenstone, of earthy trap, of un- 

 changed amygdaloids, and of amygdaloids of which the 

 superficial nodules at least have been washed out during 

 their previous exposure to mechanical action. All these 

 appear in no respect altered in structure, but resemble 

 perfectly the same modifications as they are at present 

 found in their original situations. 



Further, if the point of contact between the cement 

 and the pebble be examined, no change either in the 

 one substance or the other, is found at the planes of 

 junction. Except in some rare instances, the union 

 between the two is slight and easily dissolved. The 

 influence of the elements or mechanical violence readily 

 produce this separation, since in Canna the pebbles are 

 washed from their bed by the action of the sea. 



The last appearance worthy of notice is the condition 

 of the vegetable substances entangled in these con- 

 glomerates. I have shown on a former occasion, when 

 treating* of the relation between the bitumens and the 

 products of wood subjected to the action of fire, that 

 the changes which the latter underwent from this agent 

 were such as to convert it into a substance, which 

 however resembling bitumen to the eye, was yet es- 

 . sentially and chemically different. I have also shown 

 in the same place, that there is a connected chain of 

 evidence and analogy to prove that vegetable matter is 

 . converted into bitumen by the action of water. I may 



* Transactions of the Geological Society. Vol. II. 



