Sac ON THE ANCIENT GRAUWACKE 
longer axes ; that, in general correspondence with the average 
color of the rocks of the district, it was of a deep gray color; 
and that its boulders were chiefly of Silurian rocks, charged 
with the characteristic fossils of the system. 
Quitting the upper part of the hill, with its richly fossilifer- 
ous quarries, and striking downwards to the west and south, 
I passed through a series of corn-fields, and, on reaching a 
little stream which flows through a valley nearly parallel to 
that of the Girvan, found the rock partially exposed along its 
course. It consists here, not, as in the hill above, of an indu- 
rated sandstone, but of a gray rubbly shale, much broken by 
transverse drives and fractures, and which, though its relations 
to the sandstone are not clearly seen, seems, as premised by 
Sir Roderick Murchison, to rest over them. In localities to 
the south of the Girvan, as at Ardwell, Piedmont-Glen, and 
Peawhapel-Burn, it is rich in orthoceratites, and contains 
numerous beds of graptolites ; in this locality, known as the 
farm of Drummuck, it abounds in trilobites. Mr. M’Callum 
had wrought out the exposed front, to which he introduced me, 
until arrested by a field-fence, on which he was not permitted 
to encroach. But though I could procure, in consequence, no 
specimens for myself, I acquired several very interesting ones 
from Mr. M’Callum, which, with many others, he had dug out 
of this front; and I certainly at one time never expected to 
see a group of these curious crustaceans at once so ample 
and so characteristic, from the Grauwacke slates of Scotland. 
Calymenit Blumenbachii, the well-known Dudley trilobite, is 
the prevailing form of the deposit,—a form abundant, as its 
common English name testifies, in the lower deposits of the 
Upper Silurians of England, but which in North Wales also 
occurs in the Lower Silurians. I found, associated with this 
trilobite, in the same fragment of shale, what seems to be a 
Forbesia, — hitherto exclusively an Irish form ; and was lucky 
