Tue Birack WATER OF DEE. 185 
ward into Ireland. As the rocks of the Upper Silurian Ages 
were removed the granite massifs were exposed, and these 
gave rise to a new set of physical features. 
In the area considered in this paper there are three bosses 
of granite :—1. The Loch Doon granite, where our stream 
takes its rise. 2. Cairnsmore of Fleet granite, along which 
the Upper Black Dee runs for about nine miles. 3. The 
Bengairn-Screel range which stretches along the Solway 
margin westward to Barcloy. These three granite masses 
are very much harder than the surrounding greywackes and 
shales, and consequently they stand out in great relief amidst 
the denuded greywackes. Dr. Mort in his paper on the 
Rivers of South-West Scotland points out that the ring of 
metamorphic rock which surrounds each granite area is more 
resistant than the granite itself, and consequently, as a result 
of denudation, may retain a higher altitude. He cites the 
metamorphic rock of the Merrick as being higher than the 
adjacent granite. For the same reason the metamorphic ring 
is extremely difficult for a stream to erode—it forms a power- 
ful barrier. The altitude of the northern granite outcrop 
surrounding the Cooran Lane is over 22co feet. The central 
block, Cairnsmore of Fleet granite, gives as an average 
altitude (Shaw Hill) 1253 feet; while the southern group, 
Screel Hill, close to the Solway, is only 1100 feet. Between 
each granite mass are the greywackes and shales, severely 
denuded to low altitudes. 
The Cooran Gap. 
To get a true estimate of the significance of this deeply 
eroded ancient valley now traversed by a mere trickle of 
water—the Cooran Lane—let us examine two sections. 
The valley floor of the Cooran Gap rests upon the 1000 
feet contour. It is over three-quarters of a mile wide. It 
contains three lochs, one drained south by the Cooran, and 
two tapped at the north by the Doon. Its valley walls are 
very steep and of great height. On the east, Corserine 
reaches 2668 feet, while on the west, Mullwharchar gives 
2270 feet, and the Merrick 2754 feet. This gives a vertical 
cutting of at least 1600 feet. Considering the breadth of the 
