452 CANNA. CJEOLOGV. 



this conglomerate cannot perhaps well be understood 

 without previously taking a general survey of the 

 nature of the several rocks of that character with which 

 we are acquainted. The imperfection of this sketch must 

 be excused, from our ignorance of the different circum- 

 stances under which the various trap rocks have been 

 formed. 



Those trap conglomerates which I have had" an op- 

 portunity of seeing, exhibit the following leading features. 

 They frequently contain rolled pebbles of trap alone, 

 while they occasionally admit granite, gneiss, quartz, and 

 sandstone. These are united either by a cement com- 

 posed of all these ingredients, or by trap sand, or by 

 a solid trap rock of various aspects. In other cases 

 the fragments are angular, but differ in no respect 

 from the rolled, either in their nature or in that of 

 the cement. These angular conglomerates possess in 

 some instances an uncommon degree of hardness ; such 

 that the fresh fracture of the stone displays an almost 

 uniform texture ; the conglomerated structure being 

 scarcely detected but by the unequal weathering of 

 the surface. Of this latter variety the summit of Ben 

 Nevis and many parts of Glenco present examples : of 

 the more ordinary texture there are instances in the 

 trap of Canna. 



Another modification assumed by the trap conglo- 

 merates is that of a loose accumulation of large and 

 small fragments ; a rock which alone perhaps deserves 

 the name of trap tufo, and which is so well known 

 and so common that examples of it need not be enu- 

 merated. 



It is now necessary to inquire respecting their origin, 

 since it is plain that they are exclusively attached to 

 this particular family of rocks, are independent of the 

 general conglomerates which belong to the red sandstone 

 formation, and dissimilar in other material points to all 

 the conglomerated substances. In examining the_ con- 



