During the revision of the Midlothian coalfield by the Geological Survey, special attention was directed to the plant remains occurring in the Millstone Grit, with the view of obtaining evidence bearing on the probable boundary line between the Upper and Lower Carboniferous rocks. Mr Tait collected a number of plants from certain horizons in this division of the Carboniferous system, which were submitted to Mr Kidston, F.R.S., for determination. Those from the lower part of the Millstone Grit were regarded by Mr Kidston as belonging to the Lower Carboniferous flora, and those from the upper portion, to the Upper Carboniferous flora. While carrying on his work in the field, Mr Tait found certain fossiliferous bands in the basal portion of the Millstone Grit, charged with lamellibranchs, brachiopods, and other marine organisms, and associated with Lower Carboniferous species of plants. These observations were of service in tracing the base line of the Upper Carboniferous rocks round a large part of the Midlothian basin. It ought to be clearly understood, however, that this boundary line is based solely on the evidence supplied by the plants and fishes as determined by Mr Kidston and Dr Traquair respectively.