LOWEST RED SANDSTONE. 401 



lating and protuberant veins. When the wasting 

 of such masses has proceeded to a considerable 

 length, as often happens where they are exposed 

 to the action of the sea, the intervals between the 

 veins become so deep, that a sort of cancellated 

 or cavernous structure is exhibited ; as was for- 

 merly noticed in describing the several varieties 

 of concretionary structure found in rocks. 



A spheroidal concretionary structure on the 

 small scale, is also occasionally found in the red 

 sandstone ; and this, like the former, is princi- 

 pally detected by the action of the sea. Arran 

 exhibits striking examples of this and of the pre- 

 ceding varieties. It is not improbable, that the 

 peculiar disposition of the red and white colours 

 in the mottled varieties of this rock, are the 

 result of a concretionary structure ; but, as yet, 

 I have not been able to ascertain this by the exa- 

 mination of such varieties, after an exposure 

 capable of producing the effect in question. 



In a general sense, the red sandstone must 

 be considered as formed of fragments, more or 

 less minute, of preceding rocks, or minerals.' 

 When these are of the usual size of sand, the 



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