LOWEST RED SANDSTONE, 403 



pacted into an almost uniform mass, by a cement- 

 ing medium of crystalline quartz ; and this, 

 although less frequently, sometimes takes place in 

 the secondary red sandstone. It is in these parti- 

 cular instances, that the distinction between the 

 two rocks, in a mineralogical sense, becomes most 

 difficult. As in all parallel cases, it can then only 

 be made by an examination of the geological 

 characters of the rock in question. 



Even in these instances, however, the texture 

 of this sandstone is generally more or less distintly 

 granular, or at least splintery; nor can it be con- 

 founded with those rocks, which, like granite, are 

 formed by the mutual interference of parts purely 

 crystalline. If instances of such a difficulty do 

 occur, they are extremely rare, and are limited to 

 the places where such sandstones have been found 

 in contact with granite ; a fact to be observed in 

 some parts of Scotland . 



Where a stratum of red sandstone is formed 

 of coarser materials, these are almost invariably 

 mixed with the finer sand, and the process of 

 cementation, in these cases also, is subject to the 

 same modifications. 



D D 2 



