I.—The Ordovician Volcanic Series. This series is divisible stratigraphically and petrographically into several distinct groups, the distribution of which in the district has been indicated by Mr. Marr. * ( a ) Falcon Crag Andesite Group. —These rocks are found chiefly in the country south of Keswick and eastward to the St. John’s Vale. They are pyroxene-andesites, and thin slices show under the microscope sometimes hypersthene (converted to bastite), sometimes augite, sometimes both. To the eye the rocks are pale greenish to dark grey, with a compact ground-mass, usually containing scattered minute porphyritic felspars, or more rarely abundant crystals up to ¼ inch in length. Many of the flows are amygdaloidal. There are rare flows of more basic lavas, besides some tuffs. ( b ) Eycott and Ullswater Basalt Group. —The most widely distributed group of all. The basalts, like the andesites, often contain altered hypersthene; they are sometimes rather rich in magnetite, but olivine does not occur. These rocks are usually darker and denser than the andesites, but a discrimination is not always possible in the field. Part of the rocks are porphyritic, and in some there are conspicuous crystals of felspar (bytownite or labradorite) which on Eycott Hill reach a diameter of an inch or more. Amygdaloidal varieties are found. Basic tuffs are sometimes intercalated among the lavas, and in some places there are andesitic flows included in this group. ( c ) Scawfell Tuff and Breccia Group, with Kentmere-Coniston Slate-Band. —This group presents itself under two different phases. (1) In the central ...