MINEEALOGY; WJÏH A CLASSIFICATION OF SILICATKS. 33 



related silicate^;, like loldspaTS, scapolites and micas, and the supposed replacement of 

 silica by alumina in certain amphiboles, it was suggested that the old distinction of acid 

 and base, recognized in the dualistic hypothesis, might be set aside. M, in the generalized 

 formula as then written, «(M O,,), would theu represent not only Na and Ca, but al, si, 

 bo, ti, and ta, as well, and "to this type, which is also that of the .spinels, all silicates 

 may be referred, except a certain number which, like eudialyte, sodalite and pyrosmalite, 

 contain metallic chlorids, hauyne, nosite and lapis-lazuli, which contain sulphates, and 

 cancrinite, which holds a portion of carbonate. These are respectively basic chlorids, sul- 

 phates and carbonates, and are represented by (MA)«- MCI ; (M,0,)rt. S.M.O,." etc. To 

 these should, of course, be added the basic fluorids or oxyfluorids like chondrodite and 

 topaz ; and oxysulphids like helvite and danalite. It was then said " The above formulas 

 are intended to involve no hypothesis as to the arrangement of the elements, for in the 

 author's view, each species is an individual in which the pre-existence of different species 

 that may be obtained by its decomposition cannot be asserted." The importance of this 

 notation, proposed in 1854, will be apparent when we come to consider further the 

 question of atomic volume in its relation to mineralogical classification. 



§ 22. Another and an important question connected with the complex constitution 

 which had been assumed for silicates and carbonates was considered in the paper now 

 under review. The high molecular weight assigned to the polysilicates admitted the 

 presence therein of many atoms of base, and of partial replacements ; while the existence 

 in crystalline species of visible mixtures of foreign matters also served to explain the 

 presence of small portions of many elements detected therein by chemical analysis. It 

 had, however, become apparent that there are variations in composition which can scarcely 

 be explained in either of these ways. Delesse had already noticed that in the homœomor- 

 phous anisometric feldspars the silica-ratio varies continuously between albite and anor- 

 thite, and was disposed to regard the feldspars intermediate in composition between these 

 two as varieties only. ' Scheerer, also, had in like manner expressed the opinion that 

 the various feldspars were to be regarded as combinations of anorthite with labradorite, 

 albite or orthoclase, or of labradorite with albite. Yon Waltershausen had however given 

 a more definite shape to the notion already in the minds of chemists, when, in 1853, he pro- 

 posed to admit three typical triclinic feldspars, anorthite, albite and krablite ; the latter a 

 supposed highly siliceous species with the atomic ratios, 1 : 3 : 24, since generally regarded 

 as a mixture of albite with quartz. These three feldspars, according to him, "alone have 

 the right to be regarded as species in miniîralogy. . . . All other feldspars, labradorite, 

 andesine, oligoclase, etc., are merely mixtures of these," and were conceived by him to be 

 built up " of infinitely small crystals of anorthite and krablite, or of anorthite and albite."^ 



§ 23. At the time of writing, in 1854, I was ignorant of the lately published conclu- 

 sions of Von Waltershausen. I had then made an extended series of analyses of these 

 feldspars from the Norian rocks, and rejecting the hypothesis of Scheerer, to which I 

 referred, attempted to give the matter a more definite form by pointing out that anor- 

 thite and albite might be represented by a common formula, which, if a molecular 



' Delesse, Ann. des Mines, 1853 (5) iii. 376. Scheerer, Pogg. Ann. Ixxxix. 19, cited in L. and K. Jahresbericht 

 for 1853, p. 105. 



- Sartorius von Waltershausen, Ueber die Vulkanischen Gesteine in Sicilien und Island, Gottingen, 1853. For 

 this reference, and for other notes on the literature of this question I am indebted to my friend, Dr. G. F. Becker. 



Sec. III., 1885. 5. 



