Extrusive rocks having the chemical and mineralogical composition of the diorites are called andesites. Typical andesites are aphan itic, felsitic or porphyritic in texture and closely resemble the trachytes, the main distinction be ing that they contain plagioclase (andesine or ohgoclase) instead of orthoclase. The ferro magnesian constituent is usually hornblende, but may be either biotite or augite. The rocks are named because of their common occurrence in the Andes Mountains.
Acid andesites containing free quartz are known as dacites, a name derived from the old Roman province of Dacia, now a part of Man he Gabbros.— The gabbros are coarse- to fine-granular, granitoid rocks of plutonic origin, having a silica percentage ranging from 45 to 55 per cent. As originally defined they consisted of plagioclase (labradorite) plus a pyroxene, of the variety known as diallage; but more re cently the name has been applied to a great variety of rocks consisting of a plagioclase (at least as basic as labradorite) plus any kind of pyroxene, either monoclinic or orthorhombic. If olivine be present it is known as olivine gabbro.
The name norite is applied to that member of the gabbro family which consists of plagio clase and enstatite. If olivine be present it is called olivine-norite. The ferromagnesian con stituent may fail and the rock thus consist of labradorite feldspar alone, in which case it is known as anorthosite (from the French word for triclinic feldspar).
The accessory mineral constituents are apa tite, titanite, ihnenite, magnetite. The first and last mentioned are usually abundant, and the latter may segregate to form ore deposits.
Diabases.— 'These may properly be discussed in connection with the gabbros, for they have the same mineralogical composition. Their chief distinguishing feature perhaps is their texture, which under the microscope, if not in the hand specimen, is seen to be ophitic. This, in short, is the essential distinction between the gabbros and the diabases as ordinarily considered. In granularity they range from coarse to aphanitic. They are sometimes strikingly porphyritic, with automorphic phenocrysts of augite im bedded in an aphanitic, of augite and plagioclase (labradorite or anorthite). The
basalts or traps are the dense black aphanitic extrusive equivalent of diabase, but they grade over into the diabases by imperceptible stages. The basalts constitute great surface flows of un paralleled extent, covering thousands of square miles. On cooling they frequently break, by shrinking, into hexagonal prisms, which have their long diameters normal to the cooling sur faces. This iS known as the columnar struc ture of basalt, so famously exhibited at the Giant's Causeway, on the northeast coast of Ireland.
Olivine is a more or less constant mineral in the basalts, which have been divided into the olivine-free and olivine-bearing varieties.
Very Basic Division: Peridotites, Pyrox enites, Hornblendites and Dtutites.— Rocks of this division, as already explained, include those from which not only free quartz has disap peared, but also the feldspars, at least as notable constituents. They consist essentially of a mix ture of ferromagnesian constituents plus one or more of the ores. Olivine and pyroxene (usually augite) together constitute peridotite (from the French word for olivine). If horn blende be substituted for augite the rock is known as hornblende-peridotite. If the rock consists essentially of pyroxene it is called pyroxenite; or of hornblende alone, hornblend ite, or of olivine alone, dunite. They have as accessory constituents magnetite, chromite, tnenite and apatite. Chrtintite rarely achrs in such abundance in the peridotites as to be of economic value.
A certain rare basaltic rock of the min ecological composition of peridotite, consisting of augite and olivine in a glassy ground-mass, occurs at Limburg in Kaiserstuhl, Baden. It is called limburgite. Dense porphyritic rocks con sisting of augite and magnetite in a glassy ground-mass represent the effusive member of and are called augitites.
!alnico.— Igneous rocks belonging to any of the above families may, by violent erup tions, be blown into fine particles known as ash, or if coarser as lapilli or bombs. These may accumulate and ultimately consolidate, the fine grained material constituting tuff, the coarser material being known as volcanic breccia or agglomerate. See PYROCLASI1C.