NATURAL HISTORY OF THE BANKS OF THE TAY. 
41 
field a local outcrop of conglomerate, about 600 yards wide, is crossed, 
but with this exception the sandstones extend from Caputh Bridge to 
Perth Bridge. From Perth Bridge to Inchyra, the riYer crosses the 
band of volcanic rocks of Old Red Sandstone age which form the 
Sidlaw range of hills. Lastly, from this point down to Invergowrie, 
its direction is parallel with the Sidlaws, and it runs between them 
and the corresponding range of volcanic hills of the same age which 
runs along the northern portion of Fife. In this latter stretch it 
crosses the isolated band of Upper Old Red Sandstone which crops 
out at intervals on both sides of the valley. 
There is yet another group of rocks to be met with in the valley 
of the Tay, the consideration of which has been deferred until now foi 
the sake of simplicity, namely, the trap dykes. Of these there are at 
least nine which cross the river, and which are exposed either in the 
bed of the stream or its immediate banks. Taking them in their 
order, they are as follows :— 
1. A small Dyke crosses the river at Birnam, about half-a-mile 
below Dunkeld Bridge. 
2. At Broadgreen, about a mile below Cargill. This Dyke de¬ 
flects the course of the river for about half-a-mile, the stream being 
compelled to run parallel with the dyke for that distance before it can 
effect a passage through it. 
3 and 4. Two Dykes, one coming from the north-east and the 
other from the east, merge into each other and cross the river as one 
dyke at the Linn of Campsie. The Linn, in fact, is formed by the 
passage of the water over this dyke. 
5. The Thistlebridge Dyke, about a mile below Stanley. This 
dyke is also a very conspicuous object in the landscape, both in the 
river where it is exposed at intervals above the surface of the water, 
and also on both the banks, where it stands out in bold wall-like 
masses. 
6 and 7. Two smaller dykes may be observed in the bed of the 
river a few hundred yards below Thistlebridge. 
8 and 9. Other two dykes cross the river at Stormontfield and 
Waukmill respectively. These are not so conspicuous in the river 
itself as those farther up, but they are exposed on both banks not 
many yards from the river. 
The dykes of the Stanley district have been very fully described 
by Mr. R. Dow in a paper on “Tertiary Dykes of the Lower Tay 
Valley” (see Trans.^ VoL I., p. 226). It will therefore be sufficient 
to state here that they are all of Miocene or Middle Tertiary age, and 
form part of the great system of volcanic phenomena exemplified in 
