1848.] NICOL ON THE SILURIAN ROCKS OF THE TWEED. 201 
the vicinity, must therefore have been derived from a distance. The 
facts still seem sufficient to prove that at the time of the deposition 
of the red sandstone, the greywacke mountains were already dry land 
formed into a system of hills and valleys like those now existing, and 
traversed by rivers flowing nearly in the channels of our present 
streams. Hence these facts lend confirmation to the theory, often 
proposed on other grounds, that these mountains formed the land on 
which grew part at least of the vegetation entombed in the coal for- 
mations of England and Scotland. If we further admit, as many ap- 
pearances in this district seem to require, that the chief agent in 
forming the valleys has been running water, the period during which 
it has continued as dry land, previous to the deposition of the red 
sandstone, must have been very considerable, and we shall thus be 
compelled to carry back the formation of the greywacke to a very 
remote date. This view is confirmed by the great denudation which 
the red sandstone has evidently undergone, showing that it at one 
time covered up these mountains to a considerably greater extent. 
From the mineralogical character of these rocks, especially as 
locally developed, some interesting conclusions may be deduced. 
From the great uniformity of the formation along a space of about 
150 miles in length m Scotland, not to mention its continuation in 
other countries, we may conclude that the greywacke was deposited 
in the open sea rather than in a limited bay or gulf. But this uni- 
formity is notunbroken. In tracing this formation from its northern 
border in Peeblesshire and the Lothians, south through Selkirk- and 
Roxburghshires to the confines of England, or more than forty miles 
across the strike of the beds, I have observed that the coarser varie- 
ties of rock predominate in the north in large irregular masses and 
with a less distinct stratification ; whereas on the south the finer 
varieties of rock preponderate, in thinner, more regular, and more 
distinctly stratified beds. The fragments of quartz too are on the 
north larger and more angular. Hence the conclusion does not seem 
very remote, that the materials forming this deposit have been derived 
from the north, and deposited in a sea becoming deeper to the south. 
The fragments of quartz and the scales of mica would also seem to 
have travelled farther and to have been longer subjected to attrition, 
than the clay-slate and imbedded masses of greywacke which have 
had their origm more close at hand. 
Admitting therefore that the formation is composed of materials 
conveyed from the north, it may be mquired what were the previous 
rocks from whose destruction these beds were formed and of whose 
disintegrated materials they consist. The quartz, felspar and mica 
are evidently the components of granite, and the associated gneiss and 
mica-slate found in the north of Scotland, so that here it might be 
supposed was the true source of the materials of the transition rocks. 
It is, however, a remarkable fact, that whilst, as already stated, frag- 
ments of clay-slate and greywacke are not uncommon amongst the 
conglomerate or coarser varieties, not one of granite or of any of these 
crystalline schists has ever been observed. There is also a far larger 
proportion of clay-slate, or fragments of argillaceous rocks, than would 
be furnished by the decomposition of the primary formations of the 
