CANNA. GEOLOGY. 451 



connected with this view of its structure, arid the descrip- 

 tion was therefore reserved to this place. It is found 

 in extremely thin laminae accompanied by shale, and 

 generally involved in, or covered by, the conglomerate. 

 Similar appearances occur in Sky, but in that island, as 

 well as in Egg, it is evident that the coal appertains 

 to the lias strata. No indications of those strata are 

 found in Canna, unless this may be considered as such ; 

 nor, as far as I observed, are any fragments of white 

 sandstone or of limestone contained in the conglomerate. 



I must now remark that the union of the rounded trap 

 nodules with the substance in which they are imbedded 

 is so slight, that they are disengaged by the sea in pro- 

 portion as it gains access to them. It is plain that they 

 owe their origin to previous rocks of trap which have 

 been disintegrated; the fragments having been rolled 

 in water long before they were re-united into their present 

 situation. Of this, there is ample evidence in their struc- 

 ture ; as, independently of their rounded form, they often 

 consist of amygdaloids from which the imbedded crystals 

 have been more or less completely washed away, leaving 

 them in a cavernous state. The bed of loose pebbles 

 thus deposited appears to have been subsequently ce- 

 mented together by a fresh addition either of solid or 

 loose matter of the same nature, on which distinct beds 

 of columnar or amorphous trap have been superimposed. 

 To these succeed the various irregular alternations already 

 mentioned either of recomposed rock, or of compact trap, 

 or of columnar basalt. 



Here therefore, there is evidence of at least one epoch 

 prior to those in which the superincumbent beds have 

 been deposited, and involving a time considerably remote 

 from that in which the first of these must have been 

 formed : a period also subsequent to a preceding de- 

 struction of considerable rocks of the same substance, 

 and consequently to a far preceding formation. But 

 the peculiar difficulties which attend the formation of 



