OF THE FORTH DISTRICT^ 
425 
traversing with rapid currents the old red-sandstone^ as ex- 
pressed on the annexed Map (Plate XI.) enter the margin 
of the clay about one mile before they join the Forth. The 
former transmits about as much water as the Forth, the 
latter considerably more than that river *. 
The district of the Forth may be divided into upper 
and lower, separated by the gorge at Stirling, formed by 
approximation of the hills, narrowing the carse-ground or 
valley to one mile. The rocky substratum of the upper 
district consists, probably throughout, of the old red-sand- 
stone, which, dipping southwards, is lost under the clay 
and subordhiate gravel, which occupy the valley. This 
arrangement is presented by the sections formed by the 
Teath, Allan, and Forth, and tributary streams ; also at ex- 
cavating the ornamental river, and in forming the elevated 
aqueduct at Blair-Drummond, passing from the higher 
marginal gravel, and proceeding into the clay of the valley. 
This extensive upper district presents to the eye one 
uniform plain, the central parts occupied with mosses, 
whereof large portions have been floated off to the river 
by means of water introduced for that purpose, into which 
the peat-earth is thrown, so as to leave the clay fit for the 
purposes of agriculture. During these and other operations 
the nature of these mosses and subsoils has been unfolded. 
The clay of this district, presenting a uniformity of surface, 
is of varied quality. In so far as exhibited, by a very few 
deeper operations, its depth does not exceed twenty feet. 
Stones and pebbles of any description are unknown through 
its whole extent, but it reposes on small gravel, sand, red 
* This map is formed on the principles pointed out in a paper by the 
President of the Society, printed in Vol. I. of their Transactions ; the ex- 
terior line, of a deeper tint on the margin of each colour, expressing that 
the particular stratum reposes on the one contiguous. 
