TRANSACTIONS OF THE SECTIONS. 103 
Loch Seriden, Isle of Mull. It immediately occurred to Mr. Barstow, that advantage 
might be taken of his discovery for the interest of the estate, and I was appointed 
by him to visit professionally the deposit, and to report on its probable quantity and 
quality. I accordingly went, and arrived about the beginning of July. 
The locality of this deposit is about two miles from the head of the Loch. It is 
bounded on the south by the sea, from which it rises by a pretty rapid ascent to the 
north, The plumbago occurs in detached masses from one or two inches in dimen- 
sions to a foot or eighteen inches, imbedded in the rock, no vein having yet been 
discovered. Neither are the including masses continuous, but isolated, and every- 
where surrounded by trap rocks of various kinds, which constitute the great forma- 
tion of the island. The extent of the plumbago-bearing rock cannot, in the present 
state of the surface, be ascertained, much being covered by svil and vegetation, and 
much probably overlaid by trap; but may safely be estimated at half a mile square 
in horizontal direction, and in thickness from the level of the sea, gradually rising to 
at least 150 feet above it ; and it is extremely probable that much is overlaid by trap 
beyond those limits. 
Having directed the proper operations to be made, the result has at once happily 
justified expectation. At one of the blasts, a large mass was thrown out (now upon 
. the table) weighing above thirty pounds, while the fragments of the same mass weigh 
as much more ; but it 1s of much more importance to observe that the quality is of 
that description of which the finest drawing-pencils are made. 
On the Geological Structure and Relations of the Frontier Chain of Scotland, 
By the Rev. Professor Sepewicx, F.R.S. &e. 
§ I. General Remarks on the Chain. 
This chain, as is well known, extends from St. Abb’s Head, on the east coast, to 
the Mull of Galloway, on the west coast of Scotland; and it reappears, with iden- 
tical mineral composition and in the line of its strike, on the east coast of Ireland. 
Whatever, therefore, may be proved respecting the general relations of the chain in 
Scotland, must apply to, at least, a considerable portion of the prolonged chain in 
the north of Ireland. 
Leaving out of account all igneous and intrusive rocks, the chain is essentially 
composed of a peculiar form of grauwacke, often coarse and sometimes, though 
rarely, passing into a very coarse conglomerate, not unlike some of the conglomerates 
among the old rocks of South Wales. These hard, coarse beds alternate indefi- 
nitely with a peculiar soft, earthy and often pyritous alum-slate, which frequently 
has undergone such compression and induration that it passes into an earthy flag- 
stone, and, more rarely, into a pretty good roofing-slate; but in no instance is the 
slaty structure distinct from the stratification in any quarries that are worked for 
use. 
From one end of the chain to the other the beds are highly inclined, strike gene- 
rally in the mean direction of the chain, and are thrown into contortions and undu- 
lations. The great protruding granitic masses never form any true mineralogical 
centre, though producing, as might be expected, considerable local derangements, 
local changes of structure ; and, in a few instances, they are accompanied, near their 
junction, with the phenomena οἵ mineral veins. The axis of the chain, the centre of 
the vast undulations, seems to be very ill defined ; and the difficulty of determining 
this point is greatly increased by the bogs and extensive vegetable accumulation by 
which the sections are much covered. 
The author crossed the chain in 1841 with Mr. J. Carrick Moore (now one of the 
Secretaries of the Geological Society of London); and in 1848 he again crossed it, 
both on the western coast line and on two other traverses, in the hopes of determi- 
ning (on the principles laid down by Professors Rogersof America) the line from which 
the undulations had originated. He also (being himself crippled in 1849) employed 
Mr. John Ruthven in making out a detailed section on the line of the railroad from 
Carlisle to Edinburgh (carefully noting the steep sides of the successive undulations) 
and in collecting fossils. The evidence of the coast sections of St. Abb’s Head, the 
author has not examined personally since 1830, and dares not therefore pronounce 
