280 Dr. Mac CuUoch on the Junction 



scription ; but the rock approaches so nearly in general aspect 

 to the finer conglomerate, or gravel-stone, of the strata which 

 consists of the same materials, that, in the detached specimens, 

 it is at first sight difficult to know which is the substance under 

 examination. 



Another remarkable variety contains schistose clay, which is 

 either in small particles, or in larger, resembling fragments, or 

 else merely communicates a grey tinge to the stone. These 

 specimens similarly approximate in character to that variety of 

 the sandstone which contains small fragments of shale. 



The hesitation and uncertainty produced in examining these 

 specimens, particularly when found detached and taken up 

 from among the surrounding fragments, is even greater than 

 that arising from the contemplation of the geological appear- 

 ances before described. If the mind is turned towards the 

 consideration of the granite at that particular moment, they 

 are placed with the granite series ; if, on the contrary, the 

 sandstone has been the object last contemplated, we feel in- 

 clined to arrange them in that division. And yet, on a very 

 minute and careful examination, I doubt not that every one 

 who is habitually and extensively conversant with rocks under 

 all their uncommon modifications, will assign to each specimen 

 its true place; from internal, and if slight, yet essential, cha- 

 racteristics, which the experienced eye can discriminate, but 

 which the pen cannot describe. 



With respect to the mineralogical characters of the sand- 

 stone, I shall limit myself to those varieties which are found at 

 the immediate junction. These are the only ones that are 

 interesting in the present case, and the description of others 

 would unnecessarily extend this paper. 



The first stratum at the only part of the junction which is 

 perfectly accessible, is a very compact, reddish sandstone, re- 

 sembling quartz rock, consisting chiefly of quartz sand, with a 

 few small fragments of felspar. This graduates into a sand- 

 stone of the same colour, without fragments. Then follows a 

 purple schist, which is in some places arenaceous, in others 

 fine and fissile, succeeded by a blue shale resembling grey- 

 wacke. After this there occur, in irregular order, a series of 

 sandstones, red, purple, blue, and brown, internjixed with the 



