222 LOWEST, OR OLD RED, SANDSTONE. 



with the history of its geological origin. As far as it 

 is safe to generalize from a few facts, the number and 

 species of the ingredients depend on those of the near- 

 est primary rocks; though this rule is attended by 

 many exceptions. Yet the places where it does hold 

 good, justify the inference, that in these at least, the 

 materials have not undergone any very distant trans- 

 portation. That they have sometimes experienced 

 this, is unquestionable; as is proved by a composition 

 less coincident with that which might thus be inferred, 

 and by the form of the materials. Organic remains 

 are rare in this sandstone ; but terebratulae, rnadrepo- 

 rites, trochites, and others have been observed, as for- 

 merly mentioned. 



There are two modifications of the form of the ma- 

 terials, where of a certain size ; being either angular, 

 or rounded, and these marking respectively, the quan- 

 tity of motion they have undergone before their con- 

 solidation. The rounded form does not, however, by 

 itself, imply a distant transportation, as the long con- 

 tinued action of water will answer all the requisite con- 

 y ditions. Examples of this occur near Oban ; where 

 this form is united to a peculiarity of composition 

 marking an origin from rocks at no great distance. 

 Occasionally the size of the fragments is such as to 

 amount to three or four tons in weight; but, more 

 commonly, they do not exceed a few pounds, descend- 

 ing to the finest sand, which is indeed the predomi- 

 nant form of the fragments; as the fine sandstones far 

 exceed the conglomerates, thus giving a name to the 

 whole. 1 need scarcely say, that fragments of all sizes 

 occur in the same rock, and that a portion of fine sand 

 is essential to the structure of every variety. As to the 

 relative position of the finer and coarser strata in any 

 given deposit, there is no absolute rule, though it is far 



