AND THE ADJOINING DISTRICT, BERWICKSHIRE. 4] 
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 hill 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 variety 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 porphyry, having a basis 
of cream-coloured felspar, with disseminated small crystals. Of this Blackerstone 
hill is composed; and it may be seen at intervals between the east side of the 
granite and Hoardweel, underlying the greywacke. At some places, where the 
latter rests immediately upon the porphyry, it appears to have been actually 
fused, having lost every semblance of its original stratified structure, 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 greywacke has, in the process of fusion, become blended 
with the igneous rock, forming a curious mongrel sort of compound. 
At the bend of the river, below Hoardweel, the porphyry 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 thirty, 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 
ioneous 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 
with the greywacke. The porphyry is generally of the same kind as that further 
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. is 
