National Geographic : 1892 Mar 21
Varieties of Poqrphyrite. also abundant in the groundmass. If, as seems very probable, this rock is really a Tertiary andesite, then it is related to the older camptonite in a way similar to that in which the mica trachyte of mount Catini, in Italy, described by Rosenbusch,* approaches the minettes. The remainder of these andesitic rocks must be classified principally with reference to the structure of their groundmass. This is coarsest and most granular in numbers 8 and 17, where it is almost granitic, though with but little free quartz. The former is extensively altered, and the latter, though less so, has its horn blende and part of its feldspar phenocrysts changed to brightly polarizing epidote. The remaining specimens form a series for the most part holo crystalline, though exhibiting as extreme members a few ex amples of unindividualized base. Their phenocrysts of plagio clase and hornblende are quite the same throughout. The holo crystalline groundmass, while differing considerably in fineness, is in some granular (19, 25 and 26) and in others microlitic or trachytic (34, 35, 28, 10 and 11). In number 37 a well marked flow structure is apparent in the arrangement of the little feldspar microliths. The uncrystallized character of the groundmass is most apparent in numbers 27 and 39. Number 38, aside from being a typical andesite like the others, possesses an additional interest on account of containing numer ous rounded grains of porphyritic quartz surrounded by absorp tion halos or zones, like those described by Mr J. S. Diller in basalt t and by Mr J. P. Iddings in basalt and other rocks.. This rock once contained an abundance of brown hornblende, which is now mostly altered to green hornblende or chlorite. Its groundmass is hypidiomorphic and granular. The absorp tion zones, whatever they once were, consist now mostly of green hornblende and calcite. Augite-porphyrite (Labradorite-porphyrite?).- -Number 31 is at once noticeable on account of its strong macroscopic resemblance to that well-known type of labrador-porphyrite, the so-called porfido verde antico, which is so common among the Roman lapidaries, and which is now known to have been extensively quarried for ornamental purposes by the Romans at Marathonise * Neues Jahrbuch flir Min., 1880, ii, p. 206. t Am. Journ. Sci., 3d series, vol xxxiii, 1887, p. 45. + Bulletin U. S. Geological Survey, no. 66, 1890.
1892 May 15
1892 Feb 19