1887] Mineralogy and Petrography. | 661 
growths. In the hornblende porphyrites the hornblende crys- 
tals are less abundant. Thisclass 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 vari- 
ations in the optical characteristics of successive layers. The 
feldspar of the second consolidation occurs in grains, often form- 
ing the greater part of the ground-mass in which the crystals of 
hornblende and feldspar are found. In addition to these min- 
erals,a very light-colored pyroxene is present in those sheets 
which are intrusive between limestone-——The same author * men- 
tions another instance of the development in eruptive rocks of a 
schistose structure, accompanied, at the same time, by a change 
in mineralogical composition.2, 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 altera- 
tion 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 gabbro- 
schists. In the first the parallel arrangement of the constituents 
is distinct, but not so marked as to give rise to that perfect fis- 
sility 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 orig- 
inal 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 hy- 
peresthene 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 hy- 
peresthene and plagioclase are scattered in porphyritic crystals. 
the grass-green augite of the amphibole-andesite from near 
Kremnitz has likewise been isolated and examined. e 
igneous rocks of the Warwickshire coal-field, according to Pro- 
ce essor Rutley,* are syenites, andesites (English), quartzites, dio- | 
rites (both augitic and olivenitic), and tufas. . 
* Geol. Magazine, November, 1886, p. 481. 
2 Cf. American Naturalist, December, 1886, p. 1049. 
3 Zeits. f. Krystall., xii. p. 97. 
4 Geol, Magazine, December, 1886, p. 557- 
