2,32 Dr Hibbert on the Limestone of Burdiehouse, 



At the foot of the Grampians, in the geographical line which extends from Stone- 

 haven to Bute, transition strata of grauwacke or argillaceous schist appear in suc- 

 cession, which seem to have been deposited in a southerly direction over a consider- 

 able tract of space, though at present concealed beneath newer formations. This 

 continuity I have inferred from the fragments of grauwacke schist, which I have 

 found entangled in such trap-rocks of Angus, of the Lothians, of Ayrshire, or of 

 Berwickshire, as had forced their way through superimposed and newer deposits. 

 In the ridge of hills extending from Berwickshire to Galloway, grauwacke schist is 

 seen to emerge. 



The grauwacke schist has, generally speaking, a line of bearing from about S. 

 60° W. to N. 60° E., and dips to the west at very considerable angles, seldom less 

 than 50° or 60°. It is of a dark grey colour, and is either compact or very finely 

 granular. In some places it is highly laminar, and is successfully quarried for roof- 

 ing slate. It frequently passes, particularly in its upper beds, into a substance of a 

 very arenaceous and mechanical structure, which may be fairly regarded as a harder 

 sort of sandstone. The colour of the sandstone, or schist, thus induced, is more in- 

 clined to a light grey, greenish-grey, or yellowish-brown colour than to one of red ; 

 though to this rule I have seen exceptions. It is also remarkable for the quantity 

 of mica which it contains. In Ayrshire, beds of granular limestone alternate with 

 the grauwacke schist. 



That these beds of grauwacke schist will be found to contain organic remains, 

 particularly in their upper strata, I have strong reason to suspect. 



Subsequently to this formation of transition strata, the range of the Grampians 

 appears to have undergone a sudden elevation, which may explain the presence of 

 such lower beds of coarse conglomerate strata as were deposited at the foot of this 

 chain of mountains. I also suspect that about the same period commenced another 

 elevation of the ridge of grauwacke schist which extends from Berwickshire to 

 the Mull of Galloway. But if this southerly elevation may be supposed to have 

 actually commenced about the same period, I conceive that it was under circumstances 

 of far less violence, and that it was a gradual, rather than a sudden process. 



But whatever may be the exact nature of these disturbances or convulsions, it is 

 certain that, from the cessation of the grauwacke deposit may be dated the incipient 

 conversion of the considerable track intervening between the chain of the Grampians 

 and the lowland confines, into a sort of basin, which, in the first instance, would no doubt 

 have its continuity much broken by irregularities of surface. This I infer from va- 

 rious insulated and very limited deposits, which appear to me to have a date of for- 

 mation intermediate to grauwacke schist, and to carboniferous strata. The remark, 

 however, fails in applying to the district north of the Tay, where a continuous depres- 

 sion appears to have been formed, within which was deposited very considerable beds. 



The oldest beds which have succeeded to the grauwacke schist, and which re- 

 pose upon it in an unconformable position, consist of the hard, yet fissile and mi- 

 caceous sandstone north of the Tay, known under the name of the Arbroath Pave- 



