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PART III. PORPHYRITIC ROCKS. CHAPTER I. PORPHYRIES. Specific gravity, 2.49-2.60. Porphyr (Germ.) Porphyre (Fr.) Proportions of silica in quartz-porphyry . . . 70-81 per cent. „ „ in porphyrites (basic) ..59-61 „ The term ‘ porphyry ’ was originally applied to certain kinds of igneous rocks of reddish or purple tints, such as the red porphyry of Egypt, 1 but this primary signification has now given place to one in which structure, and not colour, is the guiding characteristic. A porphyry, as the term is now generally received, is a rock of plutonic origin, with a compact, or micro crystalline, felspathic base, in which individual crys tals of quartz, or felspar, sometimes with other minerals, are developed. Thus we have the following varieties:— (a) Quartziferous porphyry ; 2 a finely crystalline 1 Mr. R. Hunt, Descrip. Guide M. P. G. 3rd edit. p. 20. 2 This is the felsite porphyry of Zirkel.—Ibid. i. 530.