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BASALT AND WACKE. 



or augite and felspar, are intimately combined and finely granular, 

 they form basalt. The French geologists make a distinction between 

 the basalt in which augite prevails, and lliat which is composed of 

 felspar and hornblende ; but it is admitted that where the structure 

 is finely granular, or nearly compact, it is difficult, if not impossible, 

 to distinguish them. 



Basalt has a greenish or brownish black colour, is difficult to break, 

 and possesses a considerable degree of hardness ; it will, however, 

 yield to the point of a knife. On examination with a lens, even the 

 more compact varieties of basalt are seen to be composed of minute 

 crystalline grains ; it frequently contains yellowish grains of a min- 

 eral called olivine ; it contains also grains of iron-sand, and a con- 

 siderable portion of the black oxide of iron. Basalt is fusible into a 

 black glass, and is magnetic. The iron which it contains passes into 

 a further state of oxygenation when exposed to the air : hence ba- 

 saltic rocks are generally covered with a reddish brown incrustation. 

 Very black basalts are chiefly composed of augite. 



Soft earthy basalt, intermixed with green earth, forms the rock call- 

 ed wacke; it has frequently a greenish colour. When basalt or 

 wacke contains rounded cavities, filled with zeolites, chalcedony, or 

 calcareous spar, it forms amygdaloid.^ When the felspar greatly 

 prevails, and the texture becomes nearly compact, basalt passes into 

 the rock called phonolite or clinkstone, from its yielding a meiallic 

 sound when struck : the prevailing colour is gray and greenish gray; 

 it is fusible. Clinkstone, when it has a more earthy texture, passes 

 into the rock called by English geologists claystone. Clinkstone of- 

 ten contains imbedded crystals of felspar, and then becomes a trap- 

 porphyry, which varies in colour according to the prevailing ingredi- 

 ents of its base. Between felspar-porphyry and trap-porphyry there 

 is an almost imperceptible transition ; in the former, the base or paste 

 is felspar, nearly pure. Some felspar porphyries pass, gradually, 

 into granite, by an intermixture with quartz and mica. 



Pitchstone has a blackish green, or a nearly black colour; it is a 

 semivitreous substance, having the lustre and appearance of pitch, 

 and does, in fact, contain a portion of bitumen ; its other constituent 

 parts are the same as those of basalt ; it approaches nearly to the 

 black volcanic glass called obsidian, which is a lava suddenly refrig- 

 erated and perfectly vitrified. Pitchstone and obsidian are, some- 

 times, porphyritic. Hence, we have, on the one hand, a series of 

 rocks, (varying only in the increase of felspar, and state of indura- 

 tion,) from granular basalt to clinkstone and claystone, from clink- 

 stone to trap-porphyry, from trap-porphyry to trachyte and felspar- 

 porphyry, and from felspar-porphyry, with the further admixture of 



* The names Porphyry and Amygdaloid rather represent modes than substances, 

 and convey no precise ideas, unless the nature of the base be specified. / 



