462 THE FKESH-WATER LOCHS OF SCOTLAND 
pirated some of the head-waters of the eastern streams, the volume 
and erosive power of the east-flowing rivers were diminished pa^^i passu 
with the increase of energy of those in the west. It is an interesting 
and suggestive fact that the Northern Block is trenched by cross 
valleys running from sea to sea, the passes at the watershed being 
mostly below 800 feet level, while the mountain masses on either side 
often rise to 2000 or 3000 feet above sea-level. These topographical 
features may reasonably be accounted for by the relation of the 
obsequent streams flowing west to the old consequent rivers draining 
eastwards across the continental uplift. 
SCULPTUllE OF THE CENTRAL BLOCK (GRAMPIAN HIGHLANDS) 
In the Central Block the conditions affecting the evolution of the 
topography of the region resemble to some extent those j ast described. 
A core or axis of crystalline schists trending north-east and south- 
west, with plutonic igneous masses, forms the dominant feature, which 
is surrounded by less resistant sedimentary strata and contempor- 
aneous volcanic rocks. 
As might be expected, the remnants of the old consequent streams 
are best preserved in the centre, and along the south-east slope of 
this block, especially along a belt of country several miles broad near 
the Highland Border. This belt is composed mainly of highly 
inclined schistose grits of great durability, flanked by weaker strata to 
the north-west and south-east, and is traversed by consequent or 
transverse streams flowing towards the south-east. Within this belt 
the rivers have been occupied in deepening their valleys. There 
has been no capture from neighbouring consequents, which is in 
marked contrast with the behaviour of the same streams on the 
north-west and south-east, as will be shown in the sequel. 
North of this belt the grain of the rocks has had a powerful 
influence in modifying the drainage system, so that the Tay-Garry, 
about the centre of the block, is the only old consequent stream that 
extends back continuously into the interior of the region. The rivers 
flowing into the Moray Firth on the north-east, and the sea-lochs 
and sounds on the west, illustrate the potency of this factor in a 
remarkable manner. In the most northerly belt of the Central 
Block there are remnants of the old transverse valleys now occupied 
by obsequent streams draining into the Moray Firth. 
The plain on the south-east side of the Mojay Firth is due to the 
removal, in whole or in part, of a succession of comparatively weak 
strata separated from each other and from the underlying crystalline 
schists by planes of denudation and unconformability. The Mesozoic 
strata and the Upper Old Red Sandstone formed part of the original 
covering that has now been removed ; but the representatives of the 
