BASALT. 123 



are composed of feldspar and augite, or hornblende. It is 

 closely allied to syenite, differing frequently only in the pre- 

 ponderance of hornblende over feldspar, and the grey or 

 greenish hue thus induced. They frequently pass into each 

 other, and the celebrated head of Memnon, (or rather of 

 Barneses,) in the British Museum,* affords a striking in- 

 stance of this transition, the body being of greenstone, and 

 the head of syenite, though the whole bust is composed of 

 one continuous mass. 



In hypersthene rock, the feldspar is compact or crystalised, 

 and sometimes vitreous, and of a red or white colour. Ser- 

 pentine is composed of diallage with magnesia ; diallage rock, 

 of diallage and feldspar ; both are frequently associated with 

 greenstone, and pass into that substance. 



TEAP ROCKS. These have already been mentioned as 

 deriving their name from a Grothic term for a stair, or ter- 

 race, because they occur in tabular masses, rising one above 

 another, like terraces. They comprise the porphyries and 

 basalts in all their varieties of composition and aspect. The 

 term porphyry is derived from a Greek word, signifying 

 purple, from a variety of this stone, used by the ancients 

 for ornamental purposes, which was of a red colour. It is 

 now applied to any rock having a compact base, in which 

 distinct crystals, of feldspar or other minerals, are embedded, 

 and is usually indicated by the name of its base, as feldspar- 

 porphyry, which is by far the most prevalent, claystone- 

 porphyry, pitchstone-porphyry, &c. 



BASALT. Basalt is a term now restricted to. masses of 

 hornblende or augite, which contain titaniferous iron, and in 

 which crystals of feldspar are visible. It has a greenish or 

 brownish black colour, is difficult to break, possesses a con- 

 siderable degree of hardness, but yields to the knife. It 

 occurs both in horizontal tabular masses, and in dikes ; and 

 exhibits, in both conditions, the varieties of columnar, 

 globular, vesicular, and amygdaloidal structure. In over- 

 lying masses, the columns are usually vertical, in dikes they 

 are frequently horizontal. The celebrated experiment of 

 Mr. Gregory Watt proves that the columnar structure of 

 basalt arises from the pressure of numerous spheroids on 



* See the Vignette on the title-page. 



