LAKES IN KELATION TO GEOLOGICAL FEATURES 459 
1. The Northern Block, comprising the area to the north-west of the Great 
Glen and Loch Linnhe. 
2. The Central Block, extending from the Great Glen south-eastwards to the 
Firths of Forth and Clyde. 
3. The Southern Block, stretching from the low plain between the Firths of 
Forth and Clyde to the English Border and the Sol way Firth. 
The dominant factor that led to the sculpturing of the continental 
uplift into these individual masses was the existence in each case of a 
core of crystalline schists or older palaeozoic strata surrounded by 
younger formations with feebler powers of resistance. The former 
still constitute the elevated portions of these blocks, while the weaker 
strata have given rise to the existing plains and lowland areas. 
Shatter belts situated along lines of fault or dislocations of the strata 
have exercised a considerable influence in producing the isolation of 
these individual masses. The development of the lowland belts in 
the north-eastern areas was aided by the resuscitation of ancient 
land surfaces which had been deeply buried by younger strata. 
SCULPTURE OF THE NORTHERN BLOCK (nORTH-WEST HIGHLANDS) 
As might be expected, the remnants of the old consequent drainage 
system are best preserved in the two northern blocks. A study of an 
orographical map of Scotland shows that in the area north-west of 
the Great Glen the main valleys occupied by the original consequent 
rivers are continued to the West Coast, though now partly drained 
by obsequent streams. In such cases the cols form low passes. A 
further study of the behaviour of the watershed of the Northern 
Block tends to support this conclusion. For example, the water- 
parting bends far to the east along the valleys occupied by the main 
streams, while in the case of the tributaries it swings far to the west, 
thus showing that the south-easterly flowing streams had first possession 
of the ground. 
The geological evidence indicates that the grain of the crystalline 
schists had little if any influence in determining the trend of the 
consequent streams, which is transverse to the strike of these schists. 
An apparent exception occurs in the l^axford area of Lewisian gneiss, 
where the strike of the gneiss coincides with the trend of the valleys ; 
but the geological history of that region shows that the valley system 
must have been initiated at a time when the Lewisian rocks were 
buried under strata the strike of which ran N.N.E. and S.S.W. 
Shatter belts accompanying more or less vertical lines of fault and 
trending in two directions — one N.W. and S.E., and the other N.E. 
and S.W. — helped to some extent to produce the drainage system in 
this northern region. The lines of fracture along Loch Maree and 
Loch Inchard may be cited as instances of the former, and those 
