444 THE FRESH-WATER LOCHS OF SCOTLAND 
Much uncertainty prevails regarding the original sequence of de- 
position of the Dalradian sediments, the tectonics of the various rock 
groups and their relations to the Moine series along the chain of the 
Grampians. Various theories have been advanced to solve these 
problems, but each of them leaves difficulties unsolved. It has been 
contended that there is an ascending sequence from the Leny grits and 
Aberfoyle slates (groups 2 and 3) to the quart zite of Schichallion 
(group 11), the latter being the highest member of the series. But 
whatever may be the ultimate solution of these questions, it is clear 
that the crystalline schists of the Moine series and the Dalradian 
rock groups were affecbed by a common system of folds, the strike of the 
axial planes trending north-east and south-west. Great recumbent 
folds with an amplitude of several miles, accompanied by lines of dis- 
ruption (thrust-planes), were produced, which are a striking feature in 
the Glen Nevis and Glencoe areas. Another result has been the 
development of a fan-shaped arrangement of the folding along an axis 
extending from Ben Lawers in Perthshire to Loch Awe in Argyllshire. 
The crystalline schists of the Eastern Highlands were pierced by 
a later series of plutonic rocks comprising granites and diorites which 
now form large areas along the Grampian chain. Certain of these 
are undoubtedly of Lower Old Red Sandstone age, but the presence of 
g:ranite boulders in the basal contjlomerates of that formation in 
Argyllshire shows that some of the non-foliated granites in the High- 
lands must be of older date. 
From this brief outline it appears that the metamorphic rocks of 
the Highlands, between the Moine thrust-plane and the boundary 
fault extending from Stonehaven to the Firth of Clyde, include rock 
groups belonging to different geological periods, though linked 
together by a common system of folding. A broad mountain chain 
of metamorphic strata may have existed in the Central and Eastern 
Highlands in Cambrian and Silurian time. Some, however, contend 
that there is no conclusive evidence of the existence of such a chain 
till the period of elevation that preceded the close of Silurian time. 
SILURIAN 
The i>:eoloo;ical structure of the Silurian tableland of the Southern 
Uplands is extremely complicated, due partly to the non-fossiliferous 
character of many of the strata, partly to the inversion of the rocks 
over wide areas, and partly to the variation in the types of sedimen- 
tation, ranging from oceanic to shallow water and shore conditions. 
But by means of the vertical distribution of the graptolites Professor 
Lap worth has demonstrated the true order of succession of the strata. 
The evidence obtained in the course of the detailed examination 
of the region points to continuous sedimentation from Arenig to 
