122 
PRE-CAMBRIAN VOLCANOES 
BOOK II 
This identification may yet he shown to he correct, but must be regarded as 
still unproved. Traces of fossils (annelid-pipes) have l)een found in some of 
the quartzites, but they afford little or no help in determining the horizons 
of the rocks. In Donegal, where similar quartzites, limestones and schists 
are well developed, obscure indications of organic remains (corals and 
gi'aptolites) have likewise been detected, but they also fail to supply any 
satisfactory basis for stratigraphituil comparison. 
Essentially the schists of the Scottish Highlands ea.st ol the Great 
Glen consist of altered sedimentary rocks. Besides quartzites and lime- 
stones, there occur thick masses of clay-slate and other slates and schists, 
with bands of graphitic schist, greywacke, pebbly grit, quartzite, boulder- 
beds and conglomerates. Among rocks that have been so disturbed and 
foliated it is necessarily (lifficult to determine tlie true order of succession. 
In the Central Highlands, however, a certain definite sequence has been 
found to continue as far as the ground has yet been mapped. AVere the 
rocks always severely contorted, broken and placed at high angles, this 
secj^uence might be deceptive, and leave still uncertain the original order of 
deposition of the whole series. But over many square miles the angles of 
inclination are low, and the successive bands may be traced from hill to 
hill, across strath and glen, forming escarpments along tlie slopes and out- 
liers on the summits, precisely as gently-undulating beds of sandstone and 
limestone may be seen to do in the dales of A'orkshire. It is difficult to 
resist the belief, though it may, perhaps, be premature to conclude, that this 
obvious and persistent order of succession really marks the original sequence 
of deposition. In Donegal also a definite arrangement of the rock-groups 
has been ascertained which, when followed across the country, gives the 
key to its geological structure.^ 
In the (U’der of succession which has been recognized during the 
progress of the Geological Survey through the Central and Southern High- 
lands, it is hard in many places to determine whether the sequence that 
can be recognized is in an upward or downward direction. Two bands of 
limestone, wliich appear to retain tlieir relative positions across Scotland for 
a distance of some 230 miles, may afford a solution of this difficulty, and if, 
as is probalde, they are to be identified with the similar limestones of 
Donegal, Mayo and Galway, their assistance will thus Ije available across a 
tract of more than 400 miles. What is regarded as the lower zone of 
limestone is particularly well seen about Loch Tay ; what is believed to be 
the upper is typically displayed in the heart of Berthshire, about Blair- 
Athol. 
From under the Loch Tay Limestone a great thickness of mica-schists, 
“ green schists,” schistose grits and conglomerates, slates and greywackes, 
emerges up to the border of the Highlands. Above that calcareous band 
thick masses of mica-schist and sericite-schist are succeeded by a well-marked 
zone of qiiartzite, which forms the mountains of Ben-y-Glo and Schihallion, 
and stretches south-westward across Argyllshire into Islay and Jura. The 
’ Ocol. Survei/ Memoirs : Geology of NJV. Doimjal, 1891 . 
