The author endeavours to show that in the Old Red Sandstone period there extended across Scotland a branch of the sea or strait, whose northern shore was somewhere in the line of the mica schist rocks which extend from Aberdeen to the mouth of the Clyde, and its southern in the direction of the Greywacke rocks that run across from St Abb's Head to Wigtonshire. In this, at the earlier part of the period, there were considerable tidal currents; but when the upper beds were deposited, they were more or less completely absent, and there were present such as were chiefly due to the action of the wind.