42 TRANSACTIONS—PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
the west of Scotland by vast sheets of basalt, and in central Scot¬ 
land by innumerable Basalt Dykes, all running in an east and west 
direction. 
Such, in briefest outline, are the positions in the geological scale 
occupied by the rocks which the river Tay passes over in the suc¬ 
cessive stages of its course from the loch to the sea. 
B. — P H Y s I c A L. 
By H. Coates. 
Under this heading we have to consider, not the age of the rocks, 
nor their stratigraphical relations, but the positions which they now 
occupy, and their structure as it affects the configuration of the 
country. 
For the better understanding of this part of the subject, we will 
now divide the course of the river into sections according as its 
direction changes. Taking the directions very approximately, we 
find they are as follows :— 
1. From Kenmore to Logierait,—N.E. 
2. From Logierait to Cargill,—S.E. 
3. From Cargill to Perth,—S. 
4. From Perth to Inchyra,—E.S.E. 
5. From Inchyra to Invergowrie,—E.N.E. 
Taking these in succession, we will endeavour to find a geological 
cause for each change in the trend of the valley. 
1. From Ke7imore io Logierait. —This section affords an example 
of a true longitudinal valley, that is, its direction coincides with that 
of the plication of the rocks. Like the valley of Loch Tay, of which 
it is the natural continuation, it runs along the axis of an anticlinal 
arch, the crest of which has been worn away by denudation. This 
course was probably determined by the fact that the outer or convex 
region of a bent mass is the region of greatest tension, and therefore 
of least resistance, so that its particles would be specially liable to be 
attacked by the eroding agents. 
2. Fi'om Logiei'ait to Cargill. —This portion represents a trans¬ 
verse valley—that is, its direction is at right angles to that of the 
plication of the rocks. This course seems to be explained by the 
theory which Mr. Macnair and the present writer ventured to advance 
in a paper on the Old Red Sandstone of Perthshire, read before the 
Society in 1892 (see Trans.., Vol. I. p. 235). If the views there 
expressed are correct, then this portion of the valley owes its origin 
to the drainage from the high lands into the lake which occupied 
the midland valley of Scotland during Old Red Sandstone times. 
