Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/351

 we find that it is divided into two glens separated by a partial rocky barrier through which the river forces its way, and that the summit of this barrier is on a level with the uppermost line. But in describing the upper glen I showed that it bore the marks of a line level with the uppermost one of lower Glen Roy, and consequently both continuous with it, and produced by a common cause. It is important to remark this fragment of a line, as, on the supposition of a continuous water level it removes the boundary of the water from lower Glen Roy to a point further east. But the next eastern point which forms the present boundary of the head of upper Glen Roy is the source of the Spey, and this elevation separates the great eastern declivity of the water at this point, or the valley of the Spey, from the western one, or that of the Roy. The measurements made by the spirit level, as well as the observations of the barometer, prove that the source of the Spey is 63 feet lower than the upper line of Glen Roy. If therefore we recur to the same supposition that water could now be accumulated to that level, it is plain that it would flow easterly into the valley of the Spey, as well as into the western outlets just enumerated.

If we now turn to the remaining communication of Glen Roy, which is with the valley of the Spean, we shall find that this river flows with no great declivity from Loch Laggan to its junction with the Roy, a space of seven miles or thereabouts. The length of Loch Laggan is ten miles, and it is separated at its upper part from the valley of the Spey by a barrier of low rocks, and by a plain of nearly four miles in length, which conducts a sluggish stream into the Spey at a point about ten miles from its source. As on the east side of this barrier the waters are directed to the Spey, so on