CANNA. GEOLOGY. 455 



on, may thus be occasionally explained without having 

 recourse to the necessity of igneous injection, which may 

 still nevertheless have existed with the state of things here 

 supposed. 



The same conjecture also affords an easy explanation 

 of the phenomena of organic remains imbedded in trap 

 rocks, which, as far as I have witnessed, are always 

 contained either in the conglomerates or in those mo- 

 difications which show no decidedly crystallized texture. 

 These appearances are however in no respect incom- 

 patible with an igneous origin, since the ejection of 

 mud, and of matters apparently of no high temperature, 

 is found to take place even in volcanic eruptions. 



If it be admitted that the species of conglomerate now 

 described have resulted from the causes here conjectured, 

 there will be no difficulty in accounting for that which 

 I have pointed out as the only one properly deserving 

 the name of trap tuff. In this, fragments of all sizes, 

 but generally small, appear to be consolidated by mere 

 repose and pressure into a half formed stone, in which 

 nothing like a general cementing substance is seen, 

 nor any of the marks of crystallization that are charac- 

 teristic of the basaltic and greenstone varieties. 



Having thus attempted to show that the trap con- 

 glomerates are probably, like all other deposited rocks, 

 formed by a consolidation of abraded trap and under 

 water, a task of considerable difficulty remains, and 

 that is, to suggest an explanation of the alternations 

 visible in Canna, between these conglomerates and the 

 solid trap rocks. 



In the cases of the common transported breccias 

 there is found one, and apparently only one formation, 

 which may be referred to a single era ; its point in 

 time between the primary and secondary strata being 

 well defined. One general character equally attends 

 those rocks which are prior to it, and those which 

 are posterior. No general formation of breccia of a 



