I. I ntroduction . T he lead and zinc districts of Great Britain afford good opportunities for the study of these ores, owing to the variety of types met with and the advanced state of mining. Unfortunately, however, many interesting districts have been exhausted and abandoned, with the inevitable loss of much valuable information, which was not secured when the mines were in operation. As Franz Posepny has truly remarked, ‘Mining, indeed, constantly furnishes fresh evidences in new openings, but it destroys the old at the same time; and if these are not preserved for science before it is too late, they are lost for ever.’ The Geological Survey memoirs, however, have in some cases preserved many data which could not otherwise be obtained. The present work has been carried out on the basis of personal field-work in all the chief districts, coupled with laboratory work at the Imperial College of Science and Technology, where all the analyses tabulated in this paper were made. The lead and zinc veins of Britain are chiefly of the spathic and barytic types of Freiberg mineralogists. Calcite is the predominant gangue, while fluorspar, barytes, and quartz are also abundant. Zeolites occur locally, as at Strontian in Argyllshire, and at Glendalough (Wicklow). The galena is argentiferous to a varying degree, and rich silver-ores have been mined at Hilderston (Linlithgowshire), at Alva (Stirlingshire), and in parts of Cornwall. These veins, though small and unimportant, are of the Joachimsthal and Annaberg type. The country-rock of the veins embraces nearly all