AND THE ADJOINING DISTRICT, BERWICKSHIRE. 41 



the summit to SE. They here present a highly crystalline and syenitic appear- 

 ance ; and, in hardness, exceed even the granite itself, on the shoulder of which 

 they have been elevated to their present position. 



Associated with this granite are several varieties of porphyry, belonging to 

 the same geological epoch. These appear at short intervals in the bed of the 

 river, from the eastern boundary of the granite, as far as Abbey St Bathans, a 

 distance of about four miles. The granite of which the steep hiU opposite Cock- 

 burn-eastfield consists, is seen at the brink of the river to pass into a kind of 

 porphyry, consisting chiefly of whitish felspar, with crystals of dark coloured 

 mica. Another vaiiety of porphyry, which also occurs at a short distance from 

 the eastern margin of the granite, is of a deep red colour, derived probably from 

 the peroxide of iron. But the most common variety is a poi-phyry, having a basis 

 of cream-coloured felspar, with disseminated small crystals. Of this Blackerstone 

 hill is composed ; and it may be seen at intervals betAveen the east side of the 

 granite and Hoardweel, underlying the greywacke. At some places, where the 

 latter rests immediately upon the porphyi-y, it appears to have been actually 

 fused, having lost every semblance of its original stratified structiu-e, and being 

 divided, like the subjacent porphyry, into extremely sharp pyramids and wedges. 

 In some places, especially where it comes in contact with the whitish porphyry 

 before mentioned, the gi-eywacke has, in the process of fusion, become blended 

 with the igneous rock, forming a curious mongi-el sort of compound. 



At the bend of the river, below Hoardweel, the porphyiy is seen penetrating 

 the greywacke in the form of conformable dykes. From this place to the copper 

 mines, the channel of the river is narrowed by vertical rocks of metamorphic 

 greywacke. At the " Strait Loup" it rushes through a gorge so narrow that it 

 may in general be easily stepped over. The geological phenomena displayed here 

 are very interesting. Within a space of about fifty yards by thuty, the porphyry 

 has forced its way through the strata in eleven or twelve different places. The 

 greywacke is much hardened and contorted ; and, near the contact with the 

 igneous rock, becomes cupriferous, and abounds in quartz veins. The copper ore, 

 which is of the green and grey varieties, occurs in the schists which alternate 

 Avith the greywacke. The porphyry is generally of the same kind as that fiui;her 

 down the river ; but in some places passes into a greyish-white compact felspar ; 

 and, in others, becomes a kind of granite (the felspar, however, predominating), 

 which contains disseminated specks of iron pyrites. In some instances it forms 

 dykes, which are, to a certain extent, conformable with the greywacke ; but it 

 commonly occurs in irregular masses of small extent, lying among the disturbed 

 strata, and connected with each other by veins or dykes. 



The metamorphism of the greywacke is observed invariably to take place in 

 the vicinity of the granite and associated porphyries ; and the process can be 

 traced in a most satisfactory manner, through all its stages, in many places 



VOL. XVI. PART I. L 



