ERUPTIVE AND METAMORPHIC ROCKS. 565 



Eruptive Rocks, containing under 60 per cent, of Silica. 

 (Basic Rocks.) 

 Chief Mineral Coviponents or Lithological Affinities. 

 ''Dolerite. — Labradorite, Olivine, Magnetite, Titaniferous-iron. Augite is 



frequently present. 

 Basalt. — A more compact form of Dolerite. 

 Aitdesite. — A similar rock, but one in which Augite takes the place of 



Olivine. More or less glassy matter is usually present. 

 Nephelinite. — A Basalt in which the Labradorite is replaced by Nepheline. 

 MtcaceoHs Basalt. — A Basalt containing Magnesian Mica. 

 Leucitophyr.—h. rock allied to Basalt, in which the Labradorite is replaced 



by Leucite. 

 Diabase. — Plagioclastic Felspar (probably Oligoclase), Augite, Chlorite, 

 Magnetite, and Titaniferous-iron. This rock (named by Alex. 

 Brongniart in 1813) is regarded by some petrologists as an altered 

 condition of Basalt or Andesite. 

 Melaphyre. — A name given to Basalts of Palseozoic and Mesozoic age. 

 Melaphyres do not differ from the more recent basalts, except in the 

 fact that many of the original component minerals have undergone 

 decomposition or alteration. 

 Porphyrite. — A term which includes structural varieties of Andesite, Diorite, 

 and a Mica-trap known as Kersantite. These Porphyrites are re- 

 spectively named Augite-porphyrite, Hornblende-porphyrite and Mica- 

 porphyrite, and they are all characterized by having a Micro-crystalline 

 V or Crypto-crystalline ground mass, in which Porphyritic Crystals occur. 

 (Picrite. — Olivine, Augite, and Magnetite, or Titaniferous-iron. 

 Lkerzolite. — Olivine, Enstatite, Augite, Magnetite, or Titaniferous-iron, 

 and frequently Picotite. Picrite, Lkerzolite, and one or two other 

 rocks in which Olivine is a dominant constituent, are often included 

 under the term Peridotite. 

 6'a(5^-i?.— Plagioclastic Felspar (often Labradorite), Diallage, Magnetite, 

 etc. Olivine Gabbros are also occasionally met with. 

 ■p / Hypersthenite. — A similar rock to the above, but containing Hypersthene 

 ' \ instead of Diallage. 



Norite. — Plagioclastic Felspar, Enstatite, etc. Since the Pyroxene in 

 Norite and in Hypersthenite is rhombic, it is usual to include both 

 rocks under the name Norite. The Norites sometimes contain Olivine. 

 Diorite. — Plagioclastic Felspar, Hornblende, Magnetite, etc. 

 By the term Greenstone, Diorite (a name given by Hai.iy) is now usually 

 implied ; formerly both Dolerites and Diorites were included under the 

 name. 



The rocks in the Group marked A. often occur as lava flows 

 as well as in the form of dykes, volcanic pipes or feeders, and 

 bosses. They are consequently of an unquestionably volcanic 

 nature. The rocks in the Group marked B. do not occur as lava 

 flows, but simply as dykes or as bosses. Although they are at 

 times seen to graduate into the volcanic rocks of the Group A., 

 still they do not appear to have ever been poured out as lava flows 

 at the surface, but have probably solidified at considerable depths, 

 and are only now exposed by the denudation of a vast amount 

 of once superincumbent rock. They would therefore be spoken of 

 as Plutonic rocks in contradistinction to the Volcanic rocks which 

 solidified at or nearer to the surface ; but both groups have no 

 doubt often originated from the same foci, and may be regarded 

 as approximately similar matter, which has solidified under different 

 conditions of cooling, pressure, etc., inducing a difference in 

 physical and mineralogical character. 



