Introduction The Skiddaw Granite occurs over several detached areas of small extent in the mountain tract on the north side of Skiddaw and Blencathra, much of which is known as Skiddaw Forest, the birth-place of the river Caldew. The alteration in the surrounding Skiddaw Slate extends in every direction for two or three miles from the central mass, and is necessarily somewhat varied in character by the changeable nature of the slate, which not unfrequently contains beds of a sandy or even gritty texture. This last fact must be carefully borne in mind when we consider the metamorphism which has been effected. As in the last part of this memoir, the subject shall be dealt with under the three heads of:—I. Examination in the field. II. Microscopical Examination. III. Chemical Examination. In an Appendix short notices are given of similar researches on these and kindred rocks. I. Examination in the Field . 1. Unaltered Slate —The general character of the unaltered clay-slate is that of a bluish grey cleaved rock, seldom forming slates of any value, but generally weathering into small flakes and chips or into pencil-like fragments. The sandy or gritty beds are usually uncleaved—but are very inconstant in character, and over the area in question do not form beds of any thickness. 2. First Stage of Alteration : Chiastolite Slate —On approaching the Skiddaw-granite area from the south, as on walking up the valley between Skiddaw and Blencathra, the first trace of alteration met with is the appearance of small faint