42 DE. THOMAS STBREY HUNT ON THE 



It is to be noted that the well-defined zeolite, harmotome has as yet no corresponding 

 anhydrous silicate. Of the heulandite section, and the corresponding feldspars, orthoclase 

 and albite, it is to be remarked that orthoclase and albite are the only feldspars hitherto 

 found associated with zeolites, and the only feldspars as yet artificially produced in the wet 

 way. The observations of Whitney already noticed {§ 57) have since been fully confirmed 

 by Pumpelly, who finds orthoclase very common with the zeolitic minerals on Lake Supe- 

 rior, where its deposition is shown to be posterior to laumontite, prehnite, analcite, apo- 

 phyllite, quartz, calcite, copper and datolite ; the only species superimposed upon it 

 being calcite, chlorite and epidote, which latter also occasionally occurs between 

 laumontite and prehnite, in order of superposition." 



§ 81. We have placed at the end of the table the two hydrous silicates prehnite and 

 chlorastrolite which, from their associations, are evidently, secretions of basic rocks, like 

 the zeolites, though neither of them present the ratios for protoxyds and alumina which 

 characterize these silicates. Prehnite has no known corresponding anhydrous silicate, 

 while chlorastrolite, though a less common species, is interesting, inasmuch as it affords 

 the oxygen-ratios of the anhydrous species, epidote and zoisite or saussurite ; a fact of some 

 significance in connection with the abundance of epidote in the amygdaloids of Lake 

 Superior. It has also the oxygen-ratios of meiouite of the scapolite group, an anhydrous 

 silicate, which however belongs to a much less condensed type than zoisite, as is indicated 

 by its inferior density and hardness, and its ready decomposition by acids. I have else- 

 where discussed the relations of these two silicates, and liaA^e shewn that the density, 

 hardness, and chemical indifference of epidote and saussurite assign them a place with 

 garnet and idocrase, in the grenatide group ; while meionite, though lacking the proper feld- 

 spar-ratio between protoxyds and alumina, belongs to the feldsi)athides.'^ 



§ 82. It is to be noted that the protoxyd-bases of the zeolites and their related felds- 

 pathides are either alkalies or lime, baryta or stroutia, if we except the partially magnesian 

 zeolites, picranalcite and picrothomsonite, and iolite and its related hydrous species, 

 which, besides magnesia, include ferrous oxyd. The latter base enters also to some extent 

 into epidote and prehnite. It should also be remarked that small portions of ferric oxyd 

 are frequently found in the analyses of zeolites, amounting, in the red varieties of laumon- 

 tite to three or four, and in some natrolites to one and two hundredths. Some part of this, 

 however, is disseminated in the form of hematite, giving color to the zeolites, and recalling 

 the association alike of hematite and magnetite with zeolites, as already noticed, and a 

 similar occurrence of these oxyds crystallized in many granitic veins. 



§ 83. We next come to the hydrous silicates of lime and alkalies, Avhich we have 

 called, for convenience, the pectolitic group, and which are correlated in the accompanying- 

 table with other protoxyd-silicates having similar oxygen-ratios, chiefly magnesian, and 

 partly hydrated and partly anhydrous. We have indicated in the second column, for the 

 known silicates of the pectolitic group, the oxygen-ratios of E, Si, and H, as in the former 

 table, and have left a blank under H, where, as in the first three terms, for example, no 

 pectolitic or nou-magnesian species is known. 



The first place in the table is given to chondrodite, the most basic natural protoxyd- 

 silicate known, and remarkable for the replacement of a small and variable proportion of 



'^ See Pumpelly, Geology of Michigan, already cited § 74; also Amer. Journal Science, (1S71) iii, 254. 

 '* Chemical and Geological Essays, pp. 445-447. 



