SECTIONAL TRANSACTIONS.—C. 303 
Dr. STEVENSON BucHan.—The petrology of the Peterhead and Cairngall 
granites (11.45). 
Two groups of granites, Cairngall and Peterhead, are recognised in the 
mass hitherto known as the Peterhead granite. 
The Cairngall granite is a grey, porphyritic and slightly foliated rock. 
The Peterhead granite is a composite intrusion of red soda-granites (albite 
granites), of which three varieties are found. ‘The earliest of these is a 
coarse-grained type, rich in heavy minerals and containing abundant 
inclusions. This variety, coming in contact with the metamorphic rocks, 
has developed locally a granodiorite characterised by idiomorphic horn- 
blende and orthite. A finer-grained, more acid granite has been forced 
into the centre of the coarse type and was followed by the intrusion of dike- 
like microgranites. 
The most prominent joints and faults trend a few degrees west of north, 
which is the direction of most of the microgranite dikes, or a few degrees 
north of east, which is the direction of porphyrite and felsite dikes. 
The contact between granite and country-rock may be sharp, with no 
chilling or modification of the granite. Elsewhere a hybrid rock may inter- 
vene to form a perfect transition. 
No ores have been found in the granite, and there is little evidence of 
tourmalinisation. A small amount of tourmaline occurs in pegmatites in 
the metamorphic rocks near the contact. 
There is an aureole of metamorphism around the granites. 
The granites were intruded towards the end of the Silurian or very early 
in the Old Red Sandstone period. 
Mr. J. V. Harrison and Mr. N. L. Fatcon.—The effect of gravity on 
rocks in weathered simple folds : a factor in tectonics (12.15). 
In parts of South-West Persia the rocks were thrown into large simple 
anticlinal folds during the Alpine orogeny Later, these rocks, comprising 
massive limestones separated and succeeded by considerable thicknesses 
of incompetent strata, were subjected to differential erosion, which has 
unroofed the limestones and removed their support in places. The action 
of gravity upon the folded rigid rocks has produced strikingly abnormal 
large-scale structures, varying according to the attitude of the limestones. 
The structures include cascading dips, overturned flaps on the edges of 
the synclines, large recumbent folds, slip-faulted blocks, ‘ roof-and-wall ’ 
folds, etc. Similar features are liable to have been formed in the Laramide, 
Hercynian and Caledonian Revolutions—whenever, in fact, orogenesis had 
thrown the rocks of the earth’s crust into folds of large amplitude. Bailey 
has produced evidence of rare structures caused by steep topography in 
submarine earthquake zones, and Jefferies has indicated the fundamental 
instability of towering piles of rock and their inherent tendency to flatten 
under the influence of gravity. The evidence examined by the authors 
shows that this process tends to occur when the surface relief is of the order 
of 2,000 ft. or more in limestone fold-mountains. Formerly such structures 
have been imputed to the action of tangential or rotational forces only. 
AFTERNOON. 
Excursion to Rubislaw Quarry, Garlogie, Gask Quarry, Dunecht, 
€astle Fraser, Craigearn, Kemnay Quarry, Craigmyle and Blackburn. 
Leaders: Mr. J. K. ALLAN and assistants. 
