A Phosphatized Trachyte from Clipperton Atoll (Northern Pacific)

The following remarks are based on five rock-specimens from Clipperton Atoll, referred to by Admiral Wharton in his description of that island. One is a dark brown rock composed of porphyritic crystals of glassy felspar and a compact groundmass; the others are white or cream-coloured, and in some of these the same porphyritic felspars as in the first-mentioned rock may be observed. The examination of a series of microscopic sections shows that the rocks are all more or less altered trachytes. The brown rock is the least altered. It consists of phenocrysts of sanidine (Pl. XXIII, fig. 1) set in a groundmass of microlitic felspars and brown interstitial matter. No ferro-magnesian minerals are recognizable. The conspicuous phenocrysts of sanidine are, as a rule, comparatively free from inclusions; but the rock contains also some patches of felspar, which are so crowded with inclusions of a brown substance that the felspar-material forms only a small portion of the compound mass. An analysis of this rock yielded the following somewhat surprising result:— The phosphoric acid is present in the brown substance, which evidently represents, in a more or less altered form, the interstitial matter of the original trachyte. The microscopic sections of the white or eream-coloured rocks all show the structure of a trachyte. In some specimens the phenocrysts of sanidine have been more or less preserved, while the groundmass has been replaced by isotropic secondary material; in others the phenocrysts have disappeared, and their places have been wholly or partially filled up