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Thus, in augite, the lime may be in part replaced by portions of protoxide of iron, or of manganese, while the form of the crystal, and the angle of its cleavage planes, remain the same. These vicarious substitutions, however, of particular elements can not exceed certain defined limits.
The Rowley Hills in Staffordshire, commonly known as Rowley Ragstone, are melaphyre. This name has usually been extended to all granular mixtures, whether of hornblende and feldspar, or of augite and feldspar. The term DIORITE has been applied exclusively to compounds of hornblende and triclinic feldspar.
Both these rocks are composed of triclinic feldspar and augite with more or less olivine, magnetic or titaniferous oxide of iron, and usually a little nepheline, leucite, and apatite; basalt usually contains considerably more olivine than melaphyre, but chemically they are closely allied, although the melaphyres usually contain more silica and alumina, with less oxides of iron, lime, and magnesia, than the basalts.
These extensive sheets of water are surrounded by banks of tuff and volcanic sand, in which fragments of augite, leucite, and crystals of titanite are distributed.
Basalt is an igneous rock composed largely of augite and felspar, which are silicate crystals of calcium, potassium, alumina, etc., of which the Moonstone is a variety. The connecting link is that clairvoyance is found to be unusually active during and by means of moonlight.
The basaltic rocks are of a black colour and thinly stratified; they are generally highly vesicular, but occasionally compact; some of them contain numerous crystals of glassy feldspar and octahedrons of titaniferous iron; others abound with crystals of augite and grains of olivine.
Columnar basalt in the Vincentin. Basaltic pillars of the Kasegrotte, Bertrich-Baden, half-way between Treves and Coblentz. The columnar structure is by no means peculiar to the trap rocks in which augite abounds; it is also observed in trachyte, and other feldspathic rocks of the igneous class, although in these it is rarely exhibited in such regular polygonal forms.
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