450 CANNA. GEOLOGY. 



more peculiarly applicable. In many places they contain 

 rounded nodules, often of very considerable size, cemented 

 in the general mass. Rarely, fragments of red sandstone 

 occur among the other materials ; and, as in many other 

 instances, pieces of the carbonized wood already described 

 are sometimes met with among the other substances con- 

 tained in them. 



An interesting conclusion may perhaps be drawn from 

 the presence of fragments of red sandstone in the trap 

 conglomerate ; as well as from those which are scattered 

 on the surface of this and the neighbouring island, and 

 have doubtless been derived from the decomposition of beds 

 of that rock. This is, the probability that the whole of 

 Canna as well as Sandy isle rests on a basis of red sand- 

 stone. In all cases where conglomerates occur, they are 

 found to contain fragments of the rock on which they 

 repose, as well as of those prior in point of date and 

 more distant in position; the relative distances of the 

 several original rocks being often marked by the greater 

 or less attrition of the fragments, modified however by 

 $he durability of the substances. In this instance, as the 

 sandstone fragments in question seem to have undergone 

 little change, they are probably not far distant from their 

 original beds. This view is confirmed by the structure 

 of the neighbouring island Rum, and by that of Sky ; in 

 both of which, and particularly in the former, extensive 

 masses of trap repose on the strata of red sandstone. 

 Those strata have been shown to possess an even as well 

 as an elevated position, and to be remarkably regular and 

 persistent on the north-east line of the compass : it is 

 barely sufficient here to recall this circumstance to the 

 reader's attention, and to refer to the several maps on 

 which the strata are laid down. The conclusion is so 

 obvious as to require no further arguments, particularly 

 as it must hereafter come under review when the whole 

 group of the Trap islands will be compared together. 

 The existence of coal in this island appears also to be 



