I 
NATURAL HISTORY OF THE BANKS OF THE TAY. 43 
3. From Cargill io Perth .—This is the section of the river which 
crosses the area once occupied by the Old Red Sandstone lake, and 
now forming the flat and open plain of Strathmore. As the surface 
of this plain falls very little in any direction, a comparatively slight 
cause would be sufficient to determine the original course of the river. 
It is probable that, at the close of the Old Red Sandstone period, 
when the land began to emerge above the receding water, this area 
would be intersected by a number of sluggish streams, carrying off 
the drainage by different channels. Gradually these would become 
concentrated into main channels, as the water found the easiest pas¬ 
sage, and thus in time one main channel was worn out. 
4. From Perth to Inchyra.~T\ivs, short section of the river’s 
course is the most difficult of any to account for. In trying to solve 
the problem, the first question which meets us is, why did the river 
cut a channel for itself between Kinnoull Hill and Moncreiffe Hill, 
instead of taking the easier course round the latter, which really forms 
the south-western termination of the Sidlaw range of hills ? But if 
we suppose some initial cause which w^ould tend to develop a channel 
between these two hills, then the river would naturally adopt such a 
course, as being the most direct. From the structure of Kinnoull 
Hill it is evident that the hard beds' of porphyrite which form its 
upper surface must at one time have been continued across the valley 
of the Tay at this point, and have overlaid the beds of Moncreiffe Hill. 
But underneath these hard beds occur more friable beds of volcanic 
conglomerate and tuff. Now, if we suppose that the upper hard beds 
of porphyrite became fractured during the folding of these rocks into 
their present anticlinal arch, then an opening would be effected in 
the barrier of hard strata, and the softer underlying strata would be 
exposed to the action of erosion. The correctness of such an expla¬ 
nation would, of course, be proved, if we could discover a fault at the 
point where the river passes between the two hills. So far as I am 
aware, no fault has been traced just in the course of the river, but the 
presence of two faults at a short distance on either side—namely, at 
Cargill Wood and at Lairwell—shows that the cleavage of the rocks 
o o 
in this locality runs in the direction I have supposed. 
5. From Puhyra to Invergoiurie. —This section constitutes what 
is known as the Carse of Gowrie. The origin of this part of the 
valley has already been explained in the paper just referred to (see 
Trans.., Vol. I. p. 243). As was there pointed out, the area occupied 
by the Carse has been sunk down between two faults which run along 
the flanks of the volcanic hills on either side. The floor of the valley 
has thus been literally lowered down between two great cleavages in 
the once continuous strata. 
