ROCKS OF SCOTLAND. 327 
the continuous band of clay-slate which, running diagonally 
from sea to sea, reclines, at a steep angle on the northern side 
of the great Lowland valley of Scotland, against the flanks of 
the Grampians. And,—to conclude the purely historical por- 
tion of my subject, — in 1851 Sir Roderick Murchison contrib- 
uted a paper on the Silurian Rocks of the South of Scotland, 
accompanied with descriptions and figures of its characteristic 
fossils (especially of those of the Girvan deposits), which gives 
us to know, on certainly the highest authority, that whilst the 
true place of those apparently older members of the Lower 
Silurian system in Scotland which, represented by what are 
the first and second of Professor Sedgwick’s five great divisions, 
is, as the Professor himself observes, exceedingly doubtful, 
there can be scarce any doubt entertained, that in the deposits 
of Girvan and Kirkcudbright we possess the analogues and 
representatives of the middle and upper members of the Lower 
Silurians of England, and the lowest member of its Upper 
Silurians. For many years we have been accustomed to regard 
our Scotch Grauwackes and Grauwacke slates as remarkable 
for their paucity in organisms. Sir Roderick seems, on the 
contrary, to have been struck by their abundance, and the dis- 
tinctness with which they tell the story and exhibit the charac- 
ter of the deposits which inclose them. “ Fossils abound,” says 
this first of geologists, in describing Mulloch Hill, in the neigh- 
borhood of Girvan, “and for the most part their shells are so 
well preserved, that great was my astonishment when I cast 
my eye over the surfaces of this rock, and thought of the long 
time which had elapsed before such unequivocal and really 
beautiful Silurian types had been made known in Scotland.” 
The perusal of Sir Roderick’s paper greatly excited my curi- 
osity. I had visited, nearly seven years before, — guided by 
the descriptions of his “ Silurian System,” —the rich deposits 
of middle England, the Wenlock limestones and shales of 
