MINEEALOGY ; WITH A CLASSIFICATION OF SILICATES. S7 



is widely sepai'ated from the anhydrous dense adarnantoid orthite or allanite, to be 

 noticed farther on. While the species jiist noticed are more protobasic than the zeolites, 

 there are not wanting examples of zeolitoid species less protobasic than these. Such 

 is the curious barytic silicate, edingtonite, to wdiich analysis assigns for protoxyds and 

 alumina the ratio, 1 : 4 ; sloanite, zeolitic in habit, with a ratio of 1 : 5, and forestite, a 

 species closely resembling stilbite, to which is given the ratio, 1 : 6. The hydrous carbo- 

 silicate, cancriuite, and the sulpliatosilicate, ittnerite, which properly belong to the zeoli- 

 toids, will be noticed under the next tribe in s^ 83, 84. 



Tribe 7. — Protoperspathoids. 



§ 72. We have next to consider the Protoperspathoids, which include, beside the 

 feldsjiars and the scapolitcs, a number of other species of double silicates, chiefly alumin- 

 ous. The species of this tribe are distinguished from the preceding by their higher 

 density, superior hardness, and greater resistance to acids ; since, svhile the whole of the 

 zeolites and zeolitoids are decomposed thereby, generally with pectisation, only the more 

 basic of the protoperspathoids are thus attacked. 



The feldspars, like the zeolites, have the atomic ratio between the protoxyds and 

 alumina represented by 1 : 3, the silica in both being subject to the same variations. As 

 in the zeolites, the protoxyd bases are alkalies and lime, rarely with baryta, while magne- 

 sia and ferroiis oxyd are but exceptionally present. Unlike the zeolites they are an- 

 hydrous, or contain occasionally one or two hundredths of water. 



y 73. The feldspar family includes, first, the feldspars proper, represented by the 

 anorthite-albite genus ; secondly, orthoclase, microcline and hyalophane, near which 

 may be placed uephelite and paranthite ; and thirdly, leucite. These distinctions, as may 

 be seen from the table. No. VIII, correspond to different values of V. lolite, a ferro- 

 magnesian feldspathide, though peculiar in composition, and differing in crystallization 

 from the feldspars, agrees in volume with anorthite and albite. Eucryptite, which has the 

 formula of a lithia-anorthite, seems to differ from these in possessing a more condensed 

 molecule. The possibility of a more siliceous feldspar than albite, corresponding to the 

 supposed krablite of Forchammer, with its ratios of 1 : 3 : 24, should not be overlooked. 



The specific gravities of orthoclase and of microliue show for these species a consider- 

 able greater atomic volume than for albite and its related species, a fact which was noted 

 in 1854 by the writer as a reason for referring orthoclase to a less condensed molecule 

 than these (§ 30). Nephelite also shows a volume near that of orthoclase, as does the 

 baryta-potash feldspar, hyalophane, which has the same general atomic formula as andésite ; 

 while leucite, with the same atomic formula, has a still larger volume. 



§ 74. The history of that feldspar genus which includes anorthite and albite, has been 

 noticed at length in the second part of this paper, where was discussed the view that the 

 feldspars intermediate in composition between these, may be mixtures of two homœo- 

 morphous species. The notion was there expressed that while such mixtures are, as was 

 long since suggested, not uncommon in nature, many, if not all of these intermediate 

 feldspars are definite species (§ 32). The careful studies of the late George W. Hawes 

 have thrown much light on this subject by showing that in similar and apparently iden^ 



Sec. III., 1885. 8. 



