598 BASALT— AUGITE. [On. XXVIII. 



rocks in which either of these minerals play a conspicuous part are 

 easily recognizable. But there are mixtures of the two elements in 

 very different proportions, the mass being sometimes exclusively com- 

 posed of felspar, and at other times -largely of augite. Between the 

 two extremes there is almost every intermediate gradation ; yet cer- 

 tain compounds prevail so extensively in nature, and preserve so much 

 uniformity of aspect and composition, that it is useful in geology to 

 regard them as distinct rocks, and to assign names to them, such as 

 basalt, greenstone, trachyte, and others presently to be mentioned. 



Basalt. — As an example of rocks in which augite is a conspicuous 

 ingredient, basalt may first be mentioned. Although we are more 

 familiar with this term than with that of any other kind of trap, it is 

 'difficult to define it, the name having been used so comprehensively, 

 and sometimes so vaguely. It has been generally applied to any trap 

 rock of a black, bluish, or leaden-gray color, having a uniform and 

 compact texture. Most strictly, it consists of an intimate mixture of 

 felspar, augite, and iron, to which a mineral of an olive-green color, 

 called olivine, is often superadded, in distinct grains or nodular masses. 

 The iron is usually magnetic (oxydulated iron), and is often accom- 

 panied by another metal, titanium. The term " Dolerite " is now 

 much used for this rock, when the felspar is of the variety called Lab- 

 radorite, as in the lavas of Etna. Basalt, according to Br. Daubeny, 

 in its more strict sense, is composed of " an intimate mixture of augite 

 with a zeolitic mineral which, appears to have been formed out of 

 Labradorite by the addition of water, the presence of water being in 

 all zeolites the cause of that bubbling up under the blow-pipe to which 

 they owe their appellation." * Of late years the analyses of M. Delesse 

 and other eminent mineralogists have shown that the opinion once 

 entertained that augite was the prevailing' mineral in basalt, or even 

 in the most augitic trap rocks, must be abandoned. Although its 

 presence gives to these rocks their distinctive character as contrasted 

 with trachytes, still the principal element in their composition is fel- 

 spar. 



Augite rock has, indeed, been defined by Leonhard as being made 

 up principally or wholly of augite,f and in some veinstones, says 

 Delesse, such a rock may be found ; but the greater part of what 

 passes by the name of augite rock is more rich in green felspar than 

 in augite. Amphibolite, in like manner, or Hornblende rock, is a trap 

 of the basaltic family, in which there is much hornblende, and in 

 which this mineral has been supposed to predominate ; but Delesse 

 finds, by analysis, that the felspar may be in excess, the base being* 

 felspathic. 



In some varieties of basalt the quantity of olivine is very great ; and 

 as this mineral differs but slightly in its chemical composition from 

 serpentine (see Table of Analyses, p. 608), containing even a larger 



* Volcanoes, 2d ed. p. 18. f Mineralreich, 2d ed. p. 85. 



