196 - PROCEEDINGS OF THE GEOLOGICAL society.  ([Jan. 5, 
formations cover a space of more than 4000 square miles, or a twen- 
tieth part of the whole island, and have thus very considerable geo- 
graphical importance. In mineralogical character the stratified rocks 
present no great diversity, by far the larger portion consisting of 
greywacke, composed chiefly of fragments of clay-slate with rounded 
grains of quartz and scales of mica; occasionally mixed with a small 
proportion of chlorite and fragments of white or red crystalline fel- 
spar. Taken as a whole, it is very different in aspect from any rock 
found either in the old red sandstone or carboniferous formations of 
Scotland, so that there would be no difficulty in distinguishing them, 
even in hand specimens. The quartz seems in general rounded by 
long attrition, and the grains are rarely larger than a pea, whilst the 
clay-slate often forms fiat, angular laminze, occasionally several inches 
in diameter. Sometimes also the greywacke incloses distinct angular 
fragments of greywacke, or of a conglomerate rock, similar to itself 
in structure. As the name implies, this rock is usually of a grey or 
light bluish colour, and has an uneven, conchoidal fracture and mas- 
sive structure, with no trace of slaty cleavage. In the coarser varie- 
ties also the lamimee of deposition are by no means distinctly marked. 
Interstratified with the greywacke are numerous beds of clay-slate, 
—sometimes apparently only a finer variety of the former with its ma- 
terials more comminuted by attrition, sometimes it would appear a 
distinct rock. Its general colour is a light greyish or dark lead-blue, 
but is occasionally brewn, red or yellow, approaching to white. It 
has a fine laminar structure in a direction parallel to the planes of 
deposition ; a transverse cleavage so common im the slates of Wales 
and Cumberland being rarely observable in this district, and then 
confined within short distances, and apparently the effect of local 
igneous action. The slaty structure is not, however, a result of 
the mere fineness of the material, as in the same quarry, and in beds 
which do not differ in mineralogical character, it appears in various 
degrees, or is even altogether wanting. 
The igneous rocks connected with these strata are chiefly felspar 
porphyries of many distinct varieties. In the channel of a small 
stream near Innerleithen I counted, in a distance of about one mile, 
twenty-two distinct beds or veins of felspar porphyry, each with pecu- 
liar mineral characters, running W.S.W. parallel to the strata, which 
in this place are nearly vertical, so that the two formations have a 
thickness of at least 5000 feet. The more common varieties are, com- 
pact felspar rock of a deep brick-red colour and slaty structure; and 
a true porphyry with a basis of light red felspar often nearly yellow 
or white, and with numerous disseminated crystals of common or 
glassy felspar. To these, acicular crystals of green hornblende are 
often added ; and more rarely hexagonal prisms of black mica, or 
disseminated grams of quartz, and lastly iron pyrites, not in vems, 
but dispersed as a constituent in the mass. Rarely the rock passes 
into a distinct granite, of felspar, quartz, and mica, or hornblende. 
But as a whole these porphyries seem to differ mineralogically both 
from the granites of the primary mountains, and from the clay-stones 
of the Pentlands on the north and the Cheyiots on the south. They 
> Gaia 
