Ch. XXVni.] PYROXENE— AMPHIBOLE. 597 



constant relation between the crystalline form and structure of a 

 mineral and its chemical composition. The apparent anomaly, how- 

 ever, which threatened to throw the whole science of mineralogy into 

 confusion, was in a great degree reconciled to fixed principles by the 

 discoveries of Professor Mitscherlich at Berlin, who ascertained that 

 the composition of the minerals which had appeared so variable, was 

 governed by a general law, to which he gave the name of isomorphism 

 (from loog, isos, equal, and fiop(p7], 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 portions 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 defined 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, hav- 

 ino- the same chemical formula with variable bases. 



o 



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 characters of five at least of the most abundant simple minerals of 

 which rocks are composed. These are felspar, quartz, mica, horn- 

 blende, 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 accustom the eye to know the appearance of rocks under 

 the lens. To learn to distinguish felspar from quartz is the most im- 

 portant step to be first aimed at. In general we may know the fel- 

 spar because it can be scratched with the point of a knife, whereas 

 the quartz, from its extreme hardness, receives no impression. If both 

 minerals are crystalline, the felspar may be known by its lamellar, and 

 the quartz by its glass-like fracture ; but when they occur in a granu- 

 lar or uncrystallized state, the young geologist must not be dis- 

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

 them by the eye alone. If the felspar is granular, the blow-pipe may 

 be used, for the edges of the grains can be rounded in the flame, 

 whereas those of quartz are infusible. In order to detect the varieties 

 of felspar above enumerated, and to distinguish hornblende from 

 augite, the reflecting goniometer will often be useful, enabling the 

 mineralogist to ascertain the angle of cleavage and shape of the 

 crystal. 



The external characters and composition of the felspars are extreme- 

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



