LAKES IN RELATION TO GEOLOGICAL FEATURES 445 
Ludlow and Downtonian time, except in the north-west area, where 
local unconfoT-mabilities occur. 
The strata are arranged in a series of flexures, the axes of which 
run in a north-east and south-west direction parallel to the longer 
axis of the tableland. Frequently the folds are inverted, both limbs 
dipping in the same direction, and hence mere superposition is of no 
value in determining the order of succession of the sediments. 
Moreover, the types of sedimentation of the Llandeilo, Caradoc, and 
Llandovery strata to be found in the Central Molfat region differ 
in a marked degree from those that occur along the northern margin 
of the tableland, and particularly in the neighbourhood of Girvan. 
One prominent rock group — the lowest in the sequence — preserves, 
with rare exceptions, its uniform lithological characters throughout 
the uplands. Consisting of cherts and mudstones, the former contain- 
ing radiolaria and the latter hingeless brachiopods, they belong partly 
to Upper Arenig and partly to Lower Llandeilo time. The cherts, 
which have been formed from radiolarian deposits, and the mudstones 
indicate an oceanic phase of sedimentation. Their horizon is clearly 
defined, for they are overlain by black shales (Glenkiln) with grapto- 
lites of LTpper Llandeilo age, and they rest on volcanic rocks, containing, 
in the Girvan area, cherty mudstones and graptolitic shales yielding 
Middle Arenig graptolites. The greatest development of Arenig 
volcanic rocks occurs near Ballantrae in Ayrshire, where they consist 
of diabase and diabase-porphyrite lavas, agglomerates, and tuffs, 
pierced by various plutonic masses, including serpentine, gabbro, 
dolerite, and granite. They reappear, however, on numerous anticlines 
along the northern margin of the tableland and elsewhere throughout 
the uplands, where they are overlain by the radiolarian cherts. 
The subdivisions of the Moffat series overlying the radiolarian 
cherts and Arenig volcanic rocks established by Professor Lapworth 
in the central portion of the tableland, viz. Glenkiln shales (Upper 
Llandeilo), Hartfell shales (Caradoc), and Birkhill shales (Llandovery), 
imply conditions of deposition near the verge of sedimentation ; for 
they consist of black shales, cherty bands, and mudstones, with rare 
intercalations of coarser sediment. The total thickness of these 
divisions of the Silurian system in the Moffat region does not exceed 
300 feet, but when traced north-westwards to the margin of the 
tableland they are represented in the Girvan area by upwards of 
5000 feet of strata. The gradual increase in thickness of these 
divisions in this direction is due to the fact that the land from which 
the sediment was derived lay to the north. 
The members of the Moffat series appear at the surface in a 
series of sharp anticlines amid a broad development of younger 
sediments of Tarannon age, comprising conglomerates, grits, grey- 
