(EXTRACTED FROM THE AMERICAN NATURALIST, JULY, 1887.)

MINERALOGY AND PETROGRAPHY.'

Petrographical News.-In the August number of the Geological Magazine Mr. J. J. H. Teall describes an interesting suite of hornblende rocks which occur as intrusive sheets and bosses in the limestones and quartzites of the Assynt district in Scotland. From the description which the author gives of them, these rocks appear to be somewhat similar to the camptonite of Dr. Hawes.3 Three types are distinguished,-viz., hornblende porphyrites, diorites, and porphyrite diorites. In the last two classes hornblende is abundant in well-formed porphyritic crystals, bounded by the planes ∞, ∞P, -P and oP. Some of the larger of these crystals are so perfectly developed that when separated from the surrounding rock-mass their angles can be measured with a contact goniometer. Most of them are twinned according to the ordinary law, and many present fine instances of zonal Edited by Dr. W. S. BAYLEY, Madison, Wisconsin.

2 Geol. Magazine, August, 1886, p. 346.

3 Lithology of New Hampshire, p. 160, et seq.; Rosenbusch's Massige Gesteine, 1886, p. 333.

growths. In the hornblende porphyrites the hornblende crystals are less abundant. This class is characterized by the presence of feldspar in two generations. The porphyritic crystals are sharply outlined, and are developed in thick tables parallel to the clinopinacoid. They often show zonal banding, due to the variations in the optical characteristics of successive layers. The feldspar of the second consolidation occurs in grains, often forming the greater part of the ground-mass in which the crystals of hornblende and feldspar are found. In addition to these minerals, a very light-colored pyroxene is present in those sheets which are intrusive between limestone.The same author1 mentions another instance of the development in eruptive rocks of a schistose structure, accompanied, at the same time, by a change in mineralogical composition. The normal gabbro of the Lizard peninsula in Cornwall is intrusive in serpentine and other rocks, and is itself penetrated by dykes of epidiorite. It is composed of diallage, hornblende, and saussuritized plagioclase, with here and there a little fresh olivine. The hornblende is secondary and of three varieties,-a compact brown, a uralitic, and an actinolitic variety. The saussuritization of the plagioclase and the alteration of the original augite into hornblende appear to increase as the pressure to which the rock-mass was subjected is seen to have been greater. Generally, though not always, the alteration in the composition of the rock is accompanied by a change in its structure. The massive character of the normal rock is lost, and a secondary schistose structure takes its place. These schistose rocks the author calls flaser-gabbros, augen-gabbros, and gabbroschists. In the first the parallel arrangement of the constituents is distinct, but not so marked as to give rise to that perfect fissility characteristic of the third class. The augen-gabbros are similar in structure to the well-known augen-gneisses. These different types of structure, as well as the alteration in the original composition of the rock, the author regards as results of the action of pressure, which in some cases was so great as to give rise to faults. The hyperesthene crystals from the hyperesthene andesite of Pokhausz, Hungary, have been isolated and examined by A. Schmidt.3 The rock in which they occur consists of a dark gray isotropic ground-mass, in which the hyperesthene and plagioclase are scattered in porphyritic crystals. The grass-green augite of the amphibole-andesite from near Kremnitz has likewise been isolated and examined. The igneous rocks of the Warwickshire coal-field, according to Professor Rutley, are syenites, andesites (English), quartzites, diorites (both augitic and olivenitic), and tufas.