Ch. XXVIIL] PYKOXENE — AMPHIBOLE. 465 



erals which, had appeared so variable, was governed by a general law, to 

 which he gave the name of isomorphism (from itfo^, isos, equal, and /xop<p>i, 

 morphe, form). According to this law, the ingredients of a given species 

 of mineral are not absolutely fixed as to their kind and quality ; but one 

 ingredient may be "replaced by an equivalent portion of some analogous 

 ingredient. Thus, in augite, the lime may be in part replaced by por- 

 tions 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 cannot exceed certain de- 

 fined limits. 



Pyroxene, a name of Haiiy's, is often used for augite in descriptions of 

 volcanic rocks. It is properly, according to M. Delesse, a general name, 

 under which Augite, Diallage, and Hypersthene may be united, for these 

 three are varieties of one and the same mineral species, having the same 

 chemical formula with variable bases. 



Amphibole is in like manner a general term under which Hornblende 

 and Actinolite may be united. 



Having been led into this digression on some recent steps made in the 

 progress of mineralogy, I may here observe that the geological student 

 must endeavor as soon as possible to familiarize himself with the char- 

 acters of five at least of the most abundant simple minerals of which 

 rocks are composed. These are felspar, quartz, mica, hornblende, and 

 carbonate of lime. This knowledge cannot be acquired from books, but 

 requires personal inspection, and the aid of a teacher. It is well to ac- 

 custom the eye to know the appearance of rocks under the lens. To 

 learn to distinguish felspar from quartz is the most important step to be 

 first aimed at. In general we may know the felspar because it can be 

 scratched with the point of a knife, whereas the quartz, from its extreme 

 hardness, receives no impression. But when these two minerals occur in 

 a granular and uncrystallized state, the young geologist must not be 

 discouraged if, after considerable practice, he often fails to distinguish 

 them by the eye alone. If the felspar is in crystals, it is easily recog- 

 nized by its cleavage ; but when in grains the blow-pipe must be used, 

 for the edges of the grains can be rounded in the flame, whereas those of 

 quartz are infusible. If the geologist is desirous of detecting the .varieties 

 of felspar above enumerated, or distinguishing hornblende from augite, it 

 will often be necessary to use the reflecting goniometer as a test of r the 

 angle of cleavage, and shape of the crystal. The use of this instrument 

 will not be found difficult. 



The external characters and composition of the felspars are extremely 

 different from those of augite or hornblende ; so that the volcanic rocks 

 in which either of these minerals play a conspicuous part are easily re- 

 cognizable. But there are mixtures of the two elements in very different 

 proportions, the mass being sometimes exclusively composed of felspar, 

 and at other times largely of augite. Between the two extremes there is 

 almost every intermediate gradation ; yet certain compounds prevail so 

 extensively in nature, and preserve so much uniformity of aspect and 



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