1848.| NICOL ON THE SILURIAN ROCKS OF THE TWEED. 207 
immediately succeeded cannot in this part of Scotland be separated, 
on physical grounds, from the carboniferous sandstones, which rest 
conformably upon it in unbroken succession. The greywacke, though 
a much older formation, may thus, from its mere position, belong to 
any of the great divisions of strata below the carboniferous,—to the 
Lower Devonian, or Upper Silurian, as well as to the Lower Silurian ; 
and though the probability is in favour of the latter, further evidence 
from fossils is still required. 
2nd. The fossils collected by Lord Selkirk, from the vicinity of his 
residence at Kirkcudbright, occur on the other side of the ridge or 
chain of hills, from those found in Peeblesshire. For other reasons I 
have thought it probable that this is the more recent portion of the 
formation. Although the want of accurate physical maps, and the 
frequent interruptions and concealment of the strata, render it very 
difficult to trace their connection, yet many facts which I have 
observed indicate that strata, at least mimeralogically similar, have a 
very extended persistence in the line of their direction, or from 
E.N.E. to W.S.W. It is thus possible that the strata ranging along 
the opposite sides of the chain may represent different parts of the 
formation. In that case it is not improbable that the beds from 
which Lord Selkirk’s fossils were derived may be connected with, or 
underlie, certain greywacke beds in Liddesdale, also on the extreme 
south side of the formation, in which I have found numerous frag- 
ments of plants, not unlike the broken reeds and other imperfect 
vegetable remains seen on some carboniferous sandstones. The Wrae 
limestone would then range with certain limestones in Colmonel 
parish, which Mr. Moore informs me are also fossiliferous. 
3rd. It should also be noticed, that the Peeblesshire graptolites (G. 
Sedgwicki) are found in a quarry at aconsiderable distance from the 
other fossils, or about twelve miles in a direct line, and on a parallel 
eight miles distant measured perpendicular to the strike of the beds. 
They lie more in the centre of the formation, and thus not improbably 
in a different part of the series. The graptolites, of different species 
however, found by Mr. Moore on Loch Ryan would also fall im an 
intermediate and corresponding position, so that we may again infer 
their connection. It is thus possible that we have already three 
leading divisions of these formations sketched out, or three geological 
horizons indicated, by which some order may at length be introduced 
into this hitherto confused mass of strata. As the rocks, however, 
form several large folds or convolutions, so that the same bed which 
in one place sinks down into the interior, may in the continuation of 
the .same transverse section rise again and again to the surface, all 
reasoning from the mere position of the beds must, in the present 
state of our knowledge, be very uncertain. It may evidently be 
mere portions of the same bed, formed at one time, but under diverse 
local conditions, in distant parts of the Silurian ocean, in which the 
various fossils have been found. The chief value, therefore, that I 
would in the meantime attach to these fossils, is as proofs that or- 
ganic remains do occur in these mountains, and thus as forming an 
encouragement to further researches, which may at length enable us 
