278 Dr. Mac Culloch on the Junction 



equally well known, that a large portion of this latter county 

 consists of a sandstone, of such characters, and occupying 

 such a situation with respect to the primary rocks, that it 

 must be considered as an example of what is called the old 

 red sandstone. An account of this tract, which is in many 

 respects interestine:, may possibly form the object of some 

 future communication ; but it is unnecessary here to enter 

 into any details respecting either of these rocks, further than 

 is required for elucidating the objects of this memoir. It is 

 sufficient to say, in general, that as the sandstone is unques- 

 tionably the lowest member of the secondary strata, so the 

 granite resembles, in its geological position, all other granites 

 in Scotland, being in many, if not in every part of its extent, 

 inferior to all the primary strata, and, in most places, covered 

 by extensive tracts of gneiss. 



With respect to the mineralogical characters of both these 

 rocks, I must also add, that the granite undergoes numerous 

 variations of composition ; exhibiting, in some places, that 

 modification which is by some esteemed particularly genuine, 

 while, in others, it is found under a great number of different 

 aspects. The sandstone formation is somewhat complicated, 

 yet less so than that of the great central district of Scotland. 

 It is sometimes grey and arenaceous, of else compact and of 

 the same colour. In other places it is red, of various tints ; 

 while it is, in some situations, simple, in others argillaceous, 

 and, in a few, calcareous. Almost every where it is remarkable 

 for containing interposed beds of various argillaceous schists, 

 sometimes so abundant as to exceed in quantity the arena- 

 ceous strata, and even to occupy alone entire tracts of the 

 surface. These schists present great diversity of character. 

 They sometimes resemble common clayslate ; more frequently 

 they are like the coarser varieties, or the greywackes, which 

 accompany the fine clayslate. It very seldom happens that 

 they put on the appearance of the shales found in the white, 

 or upper sandstones, or those which occur in the red sand- 

 stone of Arran. 



I have observed, above, that the junction of the sandstone 

 strata with the subjacent mass of granite, is visible for a consi- 

 derable space ; and, where it occurs, the sandstone beds 



