An account of the succession and structure of the Borrowdale Series in the neighbourhood of Kentmere has already been given. In the present communication some additional notes are given on the chemical composition of the rocks and the characters of the flow-brecciation, which is one of the most typical features of the lava-flows. The appearances of certain of the more important characteristics of the rocks when seen in thin section under the microscope are recorded in a number of photomicrographs. The chemical and microscopical work here described was carried out in connexion with the field-work, and the present details are recorded in order to give completeness to an aspect of the subject which was perhaps somewhat neglected in the original paper. It was then felt that the establishment of the structure of the area was the matter of paramount importance. These notes should, however, be read in conjunction with the previous paper, more particularly with reference to pp. 14–30. It will be seen that the analyses set forth in the accompanying table confirm the descriptions of the lavas given in the previous paper. The Upper Rhyolites are apparently slightly less acid than the Kentmere Pike Rhyolites, while the most acid rocks exposed in the area are the quartz-porphyry intrusions. The differences already described between the andesite-lavas at different horizons are also well brought out in the percentages of silica in the analyses. The variation in the Upper Andesites east and west of the River Kent is especially well seen. Whereas