446 THE FRESH-WATER LOCHS OF SCOTLAND 
wackes, flags, and shales, which are repeated by folding over a belt 
of ground twenty miles across in the central part of the tableland. 
Along the northern margin of this belt the Birkhill shales (Llandovery) 
are replaced by coarser sediments, and are represented by grits, 
greywackes, and shales with thin carbonaceous seams yielding dwarfed 
representations of Lower Birkhill graptolites. 
The northern belt of the uplands, which stretches from the 
northern slope of the Lammermuir Hills south-westwards by Leadhills 
and Sanquhar to Loch Ryan and Portpatrick, is composed of Arenig, 
Llandeilo, and Caradoc strata, which rise from underneath the younger 
Tarannon sediments of the central region. In the northern tract 
these divisions of the system show lateral variations of the strata. 
For example, the Hartfell black shales (Lower Caradoc) undergo 
modification, and graptolites appear in thin black seams interleaved 
in flaggy shales or in dark sandy shales. The barren mudstones 
(Upper Caradoc) of the Central Moffat region are represented in the 
northern belt by grey micaceous shales (Lowther shales), greywackes, 
and grits, with lenticular masses of limestone, which, at Wrae and 
Glencotho, are associated with volcanic rocks. In like manner the 
Glenkiln shales lose their normal characters, and their graptolites are 
found in thin dark seams in sandy bands interbedded with greywackes 
and shales. 
In the Girvan region, as shown by Professor Lapworth, these 
lateral modifications of the strata are more strongly marked, for the 
Moffat series is there represented by a vast thickness of conglomerates, 
grits, greywackes, flagstones, shales, and limestones. To the north of 
the Stinchar valley, in the Girvan and Ballantrae region, the Llan- 
deilo and Caradoc rocks rest unconformably on an eroded platform of 
Arenig volcanic rocks, but south of the Stinchar valley this uncon- 
formability disappears. 
Along the southern margin of the uplands the Tarannon rocks of 
the central belt pass conformably upwards into Wenlock and Ludlow 
strata, which yield fossils characteristic of these subdivisions. 
North of the Silurian tableland, and within the area occupied by 
the Old Red Sandstone, near Lesmahagow and in the Pentland Hills, 
various inliers of Upper Silurian rocks are exposed, ranging from 
Wenlock to Downtonian time. The distinctive palseontological 
feature of these inliers is the remarkable fish fauna found in the Ludlow 
and Downtonian strata. The latter division consists of red and 
yellow sandstones and conglomerates with shales and mudstones, 
forming passage beds between the underlying Ludlow rocks and the 
overlying Old Red Sandstone, like the Downton rocks of Shropshire. 
The discovery of ostracods, phyllocarid crustaceans, eurypterids, and 
fishes — an assemblage of organic remains identical in some respects 
