OEIGIN OP CEYSTALLINE ROCKS. S3 



§ 105. The coustaucy in composition and the wide distribution of pinite show it to be a 

 compound readily formed and of great stability. Such being its character, it might be 

 expected to occur as a frequent product of the aqueous changes of other and less stable 

 silicates. It is met with in veinstones, in the shape of crystals of uephelite, iolite, scapo- 

 lite, feld,spars, and spodumeue, of each of which it is supposed to have been formed by 

 epigeuesis. Its frecj[uent occurrence as an epigenic product is one of the many examples to 

 be met with in the mineral kingdom of the law of " the survival of the fittest." It is, 

 however, difficult to assign such an origin to beds of this mineral, like those which 

 have been above described, which are probably the results of original deposition, or 

 of diagenesis. It is a characteristic of our present unnatural system of mineralogy 

 to banish to the category of doubtful species most of the substances which are supposed 

 to be of epigenic origin, and which do not ordinarily present a definite crystalline struc- 

 ture. Several mineral compounds are apparently indisposed to assume a crystalline con- 

 dition, and among these are pinite and serpentine. The latter is probably, like pinite, in 

 certain cases, a product of epigenesis, but few, we think, who have studied the mode of its 

 occurrence and distribution in crystalline limestones, will ascribe to it, in such conditions, 

 an epigenic origin. 



§ 106. Dana has compared serpentine and pinite on the ground of their physical resem- 

 blances, and has said that pinite is "an alkali-alumiua-serpentine, as pyrophyllite is an 

 alumina-talc."'^ The relations between the minerals thus compared are, however, mimetic 

 only and not genetic. A true system of mineralogical classification must not be based on 

 analogies such as these, nor on assumptions regarding water as replacing fixed bases, or 

 alumina as taking the place on the one hand of silica, or on the other of protoxyd-bases. 

 Some of the relations, suggested by formulas constructed in accordance with such assum- 

 tions, are not without interest from the point of view of theoretical chemistry, but serve 

 only to mislead the mineralogist who seeks for a fundamental and genetic system of 

 classification of mineral silicates. 



§ 10*7. The cardinal distinction is that between protoxyd-silicates and aluminous 

 silicates, based on their origin, and on the chemical relations of their respective bases. For 

 the latter class, there comes, in the next place, the consideration of the proportions 

 between the protoxyd-bases and the alumina, and the departures on either side from the 

 ratio, R : al = 1 : 3, as seen in the i-elations of those aluminous silicates with an excess of 

 E, on the one hand, and on the other, those with a deficiency of E, which are connected 

 with the simple ahiminous silicates. The above ratio of 1 : 3, which we have called the 

 normal ratio of protoxyd to alumina, is that not only of the feldspars and the zeolites, but 

 of diaspore, of the spinels, and of the crystalline aluminate of potash. The chemist will 

 not need to be reminded that this stable group is the simplest possible compound which 

 the hexatomic element, akiminium, can form with a monatamicor a diatomic element like 

 sodium or calcium, corresponding to a condensed molecule, the water-type of which 

 will be H, 0,. 



§ 108. The point of next importance, which is of special siguifiance in the aluminous 

 double silicates, is that of their greater or less condensation, or in other words, the relation of 

 their density to their empirical equivalent weight, as already pointed out in the case of the 



°* Dana's System of Mineralogy, 5th éd., p. 479. 



