Introduction During the last summer, while carrying on the Geological survey of the Lammermuir Hills, I was particularly struck with the number of felspathic dykes by which the Silurian strata of that region are intersected. It was my wish to ascertain, if possible, the probable date of these igneous rocks; and for this purpose it became necessary to determine the exact relation between the Old Red Conglomerates and Sandstones of East Lothian, and the Old Red Sandstone and Upper Silurian in other parts of South Scotland. I soon perceived, however, that the determination of this point had a much wider range than at first sight appeared, and that in truth it bore directly upon the question of the true classification of the Old Red Sandstone. The district which promised to afford the most satisfactory results was the Parish of Lesmahago, where, according to the section by Sir Roderick Murchison, an ascending series could be made out from the Upper Silurian, through the Old Red Sandstone, into the Carboniferous group. I visited Lesmahago, and mapped the boundary-lines of the Lower Old Red Sandstone and Carboniferous rocks over an area of from 80 to 90 square miles. The results of this examination, in so far as they bear on the Old Red Sandstone, form the first and principal part of the present communication. I shall next refer to the extension of the Lesmahago features to the north-east, across the area of the Pentland Hills, into East Lothian and Berwickshire, and then point out