1848.] NICOL ON THE SILURIAN ROCKS OF THE TWEED. 197 
are less crystalline than the former and more so than the latter, 
which they also far surpass in durability when exposed to the weather. 
From the trap rocks they differ mineralogically in consisting chiefly 
of felspar and not of augite or hornblende, and also in position, being 
generally disposed in beds parallel to the strata, whereas the augitic 
trap rocks run in veins crossing the strata in all directions. 
* In the central part of this district true trap rocks, as greenstone 
and basalt, are rare, compared to their abundance in the secondary 
formations, both on the south and north, where every square yard 
almost, as Dr. Macculloch has remarked, must be searched for them. 
Where they do occur it is m veins seldom above a few feet wide, 
though often extending for a great distance longitudinally, in one 
case to twenty-five miles or more*. In this region also they never 
seem to have overflowed at the surface, forming those large conical 
or tabular hills which are so numerous in all the secondary formations 
in Scotland. On the whole, they are in the transition districts 
far inferior in extent and importance to the felspar porphyries ; 
whilst immediately on entering the secondary formations they begin 
to form hills and large overlying masses, whilst porphyries, like those 
imbedded among the greywackes, disappear. This intimate associa- 
tion of certain igneous rocks with certain stratified formations is fre- 
quently observable in Scotland, and is one of those facts in geology 
the cause of which seems but imperfectly understood. 
Both these classes of igneous rocks, where in contact with the strata, 
have considerably modified the character of the latter. Near the 
trap veins the greywacke is often hardened and the slates converted 
into a very hard flinty slate; but in other cases they have produced. 
little or no change on the strata, or have only modified their colour. 
At other times they seem to have partly destroyed the consistence of 
the slates and rendered them softer, more friable, and more devoid of 
Fig. 1. 
ou 
NWN 
CLL 
lh UL. 
1 
\\ 
\ Uy l Y/ 
QUEL LLL, 
\e Son Elia 
T/ \ eS as 
HY /) 
~ 
ii 
%* This remarkable vein occurs in Roxburghshire, and consists of a dark, highly 
magnetic greenstone. I have traced it almost continuously from Hindhope at the 
top of the Kale, by Rink, Kirkton church and Hawick, to Whitslaid in Selkirk- 
shire; and on the south-east it is said to extend to the sea-shore near the mouth 
of the Coquet. It thus traverses the carboniferous formation of Northumberland, 
the porphyries of the Cheviots, the red sandstone of Roxburgh and the greywacke. 
a a. Felspar porphyry. bb. Greywacke. 
