OEIGIN OF CEYSTALLINE EOCKS. 41 



making silicates. The conditious of crystallization of mineral matter will next be consi- 

 dered in relation to the formation of rocks, after which the conclusions of our present study 

 will be briefly summed up in the fovirth and last part of this essay. 



§ 79. In the accompanying table of zeolites and related species, are placed, in the first 

 column, the names of hydrous species ; in the second column are given the oxygen-ratios 

 between the protoxyd-bases, the alumina, the silica, and the water, represented respectively 

 by E, r. Si, and H ; while in the third column, appear the symbols of the predominant pro- 

 toxyd-bases in the resijective sx^ecies. In the fourth column are given the names of corres- 

 ponding anhydrous species, the protoxyd-bases of which are too well known to require 

 designation. In this and the succeeding tables I have generally followed the terminology 

 and adopted the formulas given in the fifth edition of Dana's " System of Mineralogy." 



In the line with the most basic zeolite known, thomsonite, are placed not only the 

 feldspar, anorthite, but a scapolitic species, parauthite, and sodalite. The minerals of the 

 sodalite group, including hauyine and nosite, correspond, as is well known, to a silicate 

 of the anorthite type united with a chlorid or a sulphate. "With nephelite is coupled the 

 hydrous species gismondite, a true zeolite. The recent analyses, by Cross and Hildebraud 

 of the zeolites of Table Mountain, Colorado, give for the zeolites having the characters 

 of thomsonite a proportion of silica greater than corresponds to the formula of that mineral 

 given by Rammelsberg, which we have placed in the table. Some of their analyses, while 

 yielding almost exactly the other ratios of the formula, give for silica, instead of 4'00, the 

 numbers, 465, 4"*76 and even 5'lV ; showing a composition more silicious than that of gis- 

 mondite, and approaching that of a zeolite corresponding to fahlunite, barsowite and 

 bytownite. These chemists, while believing the specimens analyzed by them to represent 

 a pure and unmixed mineral, leave undecided the question of its real composition. 



§ 80. The feldspars, barsowite and bytownite, according to several concordant analyses, 

 are as distinct from anorthite as they are from labradorite, and apparently as much entitled 

 to form a distinct species as the latter feldspar, or as andésite or oligoclase. The composition 

 of a lime-barsowite, with the ratios, 1 : 3:5, would be silica 48.54, alumina 33.33, and 

 lime 18.13 ^ 100.00. "With these feldspathic minerals has been placed iolite, which is a 

 magnesia-iron silicate, giving the above ratios and, as I long since pointed out, is from its 

 atomic volume entitled to be regarded as a feldspathide. "With these various anhydrous 

 species would appear to correspond very nearly the so-called thomsonite of Cross and 

 Hildebrand. With this anhydrous group we have placed two hydrous magnesian species, 

 the one, esmarkite, also called praseolite and aspasiolite, and the other fahlunite, which 

 includes what have been called auralite and bousdorfhte. These species are often associated 

 in nature with iolite, from which they differ only in the presence of water, and they have 

 been by most mineralogists regarded as formed by subsequent hydration from this 

 mineral. This view, however, was contested by Scheerer, who regarded the association 

 of the hydrous and anhydrous minerals, as due to a simultaneous crystallization of two 

 isomorphous species." 



The relations of the silicates of the natrolite section to labradorite are obvious from 

 the table. The same may be said of the relations of the numerous silicates of the anal- 

 cite section to andésite, hyalophane and leucite, and of the faujasite section to oligoclase. 



" Amer. Jour. Science (1848), v. 385, from Pogg. Annalen, Ixviii, 319. 



Sec. III., 1884. 6. 



