I.—Notes on the Cheviot Andesites and Porphyrites

Before proceeding to describe the more common porphyrites, I wish to return to the subject of the true andesites, referred to in my last paper, and their relation to Tertiary and recent rocks of a similar chemical and mineralogical composition. On referring to the note appended to that paper, it will be seen that my description of the pyroxenic constituents of these rocks was very imperfect, not to say inaccurate. The number of elongated sections which extinguished parallel with the vibration-planes of the crossed Nicols attracted my attention when first I examined the rock, but seeing that the same feature occurred in the so-called augite-andesites of Hungary and Santorin, and seeing further that as regards cleavage and crystalline form the pyroxenic constituents of the Cheviot rock resembled those of the augite-andesites above mentioned, I came to the conclusion that the prevailing pyroxene in both cases was the same, and also that it was augite. In the former conclusion I still think I was right, but in the latter I was unquestionably wrong.