CUMBRAY (GREAT). GEOLOGY. 481 



although there is no doubt that these have in many cases 

 produced the appearances here noticed. An instance of 

 alluvia thus generated by the gradual descent of the 

 surface along the declivities of the mountains, was already 

 pointed out in Arran, and they are of common occurrence 

 throughout the Highlands, as they doubtless are elsewhere 

 in similar situations. They form an important modifi- 

 cation of a class of phenomena which has not yet been 

 studied with the attention it deserves. 



The veins of the great Cumbray present many different 

 varieties of composition ; but greenstone, basalt, and por- 

 phyritic traps, are among the most common. Two only, 

 appear sufficiently remarkable to claim a more distinct 

 notice, after the innumerable observations on similar sub- 

 stances which have been scattered throughout the fore- 

 going pages. One of these is conspicuous for its foliated 

 structure ; examples of which have however been pointed 

 out before. The other is traversed by numerous reticu- 

 lating veins of calcareous spar; a circumstance which 

 I have not elsewhere observed in the veins of trap, though 

 not very uncommon in the great overlying masses. It 

 is said that gypsum has been found in Cumbray, but 

 it did not occur in my researches, nor could I obtain 

 any authentic information on the subject. 



With this brief but sufficient sketch, the account of 

 the great Cumbray is terminated; but as this is the last 

 occasion on which the sandstone of the Clyde will come 

 under review, it will not be useless now to remark, that 

 our knowledge of the several deposits of red sandstone 

 occurring in Britain appears as yet imperfect. It may 

 be a question indeed whether that imperfection is not, to a 

 certain extent, rather imaginary than real, and produced 

 by the adoption of certain general characters to which 

 those of the several deposits do not in all points conform. 

 On investigating the causes of this uncertainty, it appears 

 to have resulted in some measure from the unavoidable 

 use of the term red, thus often leading observers, 



VOL. II. I I 



