Page:The Journal of geology (IA journalofgeology11893univ).pdf/184
following divisions are established under andesite: Pyroxene-andesite, hornblende-pyroxene-andesite, and hornblende-andesite. Under dacite: Pyroxene-dacite, pyroxene-hornblende-dacite, biotite-hornblende-dacite. As already remarked, the mineralogical gradation from pyroxene-andesite to biotite-hornblende-dacite is by very gradual transitions. Biotite is most abundant in the most silicious varieties.
The microscopical character and the distribution of the porphyritical minerals and of the groundmass, and the relation between the microstructure of the latter and the composition of the rocks are described in detail, and appear to be identical with those existing in the andesites and dacites of western North America. These descriptions are presented in the most satisfactory manner, but need no special notice except to call attention to the occurrence of microlites of quartz in the form of minute pyramids .003 mm. in diameter, which are an essential component of the glassy groundmasses of numerous dacites. Precisely similar microlites of quartz have been observed by the present writer in certain silicious glasses in the Yellowstone National Park, the descriptions of which have not yet been published.
The chemical composition of the rocks is shown by fifteen complete analyses and ten silica determinations. They range from 54.21 per cent. of silica to 70.22 per cent. The analyses were made from perfectly fresh and unaltered rocks, and the high percentage of water found in some cases, which reaches 3.62 per cent., is referred to the glass base. This is sometimes markedly pearlitic and hydrated. The variations in the proportions of the chemical components throughout the rock series is pointed out, and is correlated with the variations in the mineral constituents.
Attention is called to the fact that a frequent mode of alteration among these lavas leads to the development of opal, and the consequent increase in the silica percentage, so that a determination of the silica in a rock may be misleading unless the rock is known to be unaltered.