102 REPORT—1 850. 
so widely and so strikingly exhibited in Scotland. North of the Friths of Forth and 
of Tay, there is a band of old red sandstone; and a broken band of similar sandstone, 
doubtful, perhaps, in position, though by many considered also old red, occurs 
south of the Frith of Forth, the carboniferous strata between being dotted by detached 
masses of igneous rocks. Near North Berwick a small patch of the sandstone and 
conglomerate occurs, separated entirely from either of the great bands, and totally 
isolated from even the carboniferous strata by a great mass of trap rock, rendered 
remarkable from the eminences of North Berwick Law, and the Bass Rock, Proceed- 
ing from Tantallon Castle towards North Berwick, the first remarkable fact con- 
nected with this association of igneous and secondary rocks occurs on the shore 
about one mile from Tantallon Castle, where a conglomerate rock appears enveloped 
in the trap, a low ridge of which borders on each side a mass of the conglomerate, 
fifty yards wide, and about the same number of yards long. This conglomerate is 
due to igneous intrusion, and contains fragments of various sizes of the secondary 
rocks, sandstone, limestone, &c., arranged promiscuously, the larger axis of some 
being in one and of otiiers in another direction, and all manifestly much affected and 
altered by the contact with a trap rock, Leaving this locality, and advancing towards 
North Berwick, conglomerate occurs again, which has been much altered, or rather 
mixed up with igneous matter, until near North Berwick the rocks appear in their 
natural state. 
Taking these circumstances into consideration, and combining them with the 
isolated position of the secondary patch, Lieut.-Colonel Portlock is disposed to believe 
that this position itself is also due to the action of the igneous reck, which has raised 
up the sandstone and conglomerate, and disconnected them from the great body of 
the gld red sandstone. 
On the Geological Position of the Black Slates of Menai Straits, §c. 
By Professor Ramsay, F.R.S., F.G.S. 
The author briefly stated some of the details of work that led the Geological Survey 
of Great Britain to certain conclusions respecting the age of the black slates of 
Menai, &c. These slates had previously been considered by Professor Sedgwick to 
underlie the purple and green sandstones and slates of Llanberis and Harlech, and 
therefore to be the lowest member of the Cambrian group of Wales. Mr. Sharpe in- 
cluded them with a part of the Upper Silurian strata. It was now shown that they 
belong to the Lingula beds, the lowest member of the Silurian series, these gradually 
folding round the purple sandstones towards Aber, in a dome-shaped form, when 
they are cut off by a N.E. fault throwing them down on the N.W.; from thence to 
the neighbourhood of Caernarvon, interrupted by intrusion traps, and the second 
outcrop of the Cambrian purple beds near Bangor. 
He next showed that the rocks of the higher part of Snowdon, &Xc. are the equi- 
valents of the Bala limestone and its underlying volcanic ashes (proved by a series of 
outliers, &c.), the igneous portion Gf these beds prodigiously thickening out from the 
neighbourhood of Bala to the N.W., whereas the underlying igneous series of Merio- 
nethshire (Cader Idris, Aran Mowddwy, &c.) thins out and disappears before reach- 
ing the parallel of Snowdon. 
The greenstones are not of a contemporaneous origin with the ordinary strata, 
though apparently interstratified with them in the same manner as the felspathic 
traps and ashes. They were thrust in between the beds before the main disturbance, 
the whole having been subsequently redisturbed together. ‘There are other masses 
of intruded felspar trap of a granitic character of later date (Llanberis, Caernarvon, 
the Rivals, &c.), for various reasons of detail, probably all older than the old red 
sandstone. 
Notice of the recent Discovery of Plumbago or Graphite in the Island of Mull, 
Hebrides. By ALexanveER Rose. Communicated by Professor Fleming. 
This deposit of plumbago, or as it is more vulgarly named black-lead (the graphite 
of miueralogists), was discovered in the month of June last, by Charles Murray 
Barstow, Esq., of India Street, in the estate of Killimore, on the northern side of 
