Abstract

The district which forms the subject of this communication is situate immediately north of Cardiff. It is bounded on the east and west by the Rhymney and Taff Valleys, and on the north by the Carboniferous rocks of the South Wales Coalfield. The area thus denned is about 16 square miles in extent.

We began a systematic petrological investigation of the Old Red Sandstone of this district in the hope that a more detailed knowledge of the sediments would establish a clearer conception of the conditions of deposition, of the probable source of the rocks, and of their relation to the underlying and overlying stratigraphical divisions. During the course of the work certain fossil localities were discovered. These represent the first records of Old Red Sandstone fossils within the area defined above.

The Old Red Sandstone of South Wales, which crops out round the greater part of the South Wales Coalfield, is a continuation of the Herefordshire outcrop; but, in the neighbourhood of Cardiff, it differs considerably in thickness from many other parts of the Old Red Sandstone around the Coalfield Syncline, and more especially from that of the North Crop.

There is a general lithological resemblance between the Old Red Sandstone rocks of the Cardiff district and those of the North Crop, although there are certain differences at particular horizons (see p. 496).

Prof. W. J. Sollas estimated the total thickness of the Old Red Sandstone of the Cardiff area to be 4273 feet, making his measurements along a