466 BASALT— AUGITE — TRACHYTE. [Ch. XXVIII 



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 fa- 

 miliar with this term than with that of any other kind of trap, it is dif- 

 ficult 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, and is often accompanied by another metal, titanium. 

 The term " Dolerite" is now much used for this rock, when the felspar is 

 of the variety called Labradorite, as in the lavas of Etna. Basalt, ac- 

 cording to Dr. Daubeny, in its more strict sense, is composed of " an in- 

 timate mixture of augite with a zeolitic mineral which appeal's 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 Avas 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 con- 

 trasted with trachytes, still the principal element in their composition is 

 felspar. 



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. Am- 

 phibolite, 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 Analysis, p. 475), containing even a 

 larger proportion of magnesia than serpentine, it has been suggested 

 with much probability that in the course of ages some basalts highly 

 charged with olivine may be turned, by metamorphic action, into ser- 

 pentine. 



Trachyte. — This name, derived from rpa^us, rough, has been given to 

 the felspathic class of volcanic rocks which have a coarse, cellular paste, 

 rough and gritty to the touch. This paste has commonly been supposed 

 to consist chiefly of albite, but according to M. Delesse it is variable in 



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



