III.—On the Relations between the Skiddaw Slates and the Green Slates and Porphyries of the Lake-district

The Skiddaw Slates, or Lowest Silurian Rocks of the Lake-district, are succeeded upwards by a great series of ashes with interbedded traps and porphyries, to which the name of “Green Slates and Porphyries” has been applied, and it has always been believed that the relations between the two were those of perfect conformity. As early, however, as the year 1867 I was led, from certain phenomena which I had observed, to express the opinion that “whether the Green Slates are really conformable with the Skiddaw Slates is a question which admits of doubt, though data are wanting to arrive at a definite conclusion.” (Geology of Cumberland and Westmoreland, p. 33.) In the beginning of November, 1868, I discovered what seemed to be a marked want of conformity between the Skiddaw Slates and the Green Slates, the localities where this occurred being the eastern side of the mouth of the vale of St. John, the west side of Derwentwater (near Lowdore), and the mouth of Borrowdale. As these phenomena had not been previously noticed by any former observer, and as they appeared to me to be of considerable importance, I have made a systematic investigation of the subject, and have arrived at the following results.