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XI. — Chapters on the Mineralogy of Scotland. Chapter VIII* — Silicates. By 

 M. Forster Heddle, M.D., Past President of the Mineralogical Society of Great 

 Britain, Emeritus Professor of Chemistry in the University of St Andrews. 



(Read December 6th, 1897.) 



The earlier mineralogists laboured under two great disadvantages. They could not 

 readily, on account of the small number of students of chemistry, call in the aid of that 

 science : and at the time when mineralogy was becoming a distinct branch of science 

 chemistry was in itself crude as well as cumbrous. They were thus forced to rely 

 chiefly upon external properties ; and, where crystalline form was absent, they were 

 confined to what may be called physical properties alone. 



Their knowledge of the composition of bodies being thus limited and uncertain, 

 the old nomenclature was to a considerable extent founded upon external features 

 alone. 



It is the habit of many of the silicates to run out into lengthened crystals, the 

 greatest amount of their concreting material being deposited in the direction of the 

 main axis of the crystal, and when a multiplicity of crystals are concreted, these are 

 thrown out from a common centre of crystallising growth, to radiate through the 

 matrix, very much after the manner of such crystals as have grown in what we term 

 empty or free space, where no matrix is present to interfere with a tendency to diver- 

 gence. This fact, the evident displacement of that which is not now displaceable, gives 

 us, in the first place, some information as to the condition of the matrix of divergent 

 crystal groups at the time of their formation ; and leads us, in the second, to consider 

 whether that matrix was in a very different condition, or held in degree any very 

 different relationship (as a body foreign to the substance crystallising in it) from the 

 liquid or the vapour present in those cavities in which we usually find divergent crys- 

 talline groups. 



The Swedish mineralogist Wallerius, who wrote in 1747, was one of the earliest 

 authors who instituted group-arrangements. After considering the gems, and rock- 



* Chapter I. The Rhombohedral Carbonates. Part I., . . . Trans. B.S.E., vol. xxvii. p. 493. 



„ II. The Felspars. Part I., .... „ „ xxviii. p. 197. 



„ III. The Garnets, ...... „ „ xxviii. p. 299. 



„ IV. Augite, Hornblende, and Serpentinous Change, . . „ „ xxviii. p. 453. 

 „ V. The Micas ; with description of Haughtonite, a new Mineral 



Species, ....... „ „ xxix. p. 1. 



VI. "Chloritic Minerals," . „ „ xxix. p. 55. 



VII. Ores of Manganese, Iron, Chromium, and Titanium, . „ „ xxx. p. 427 

 VOL. XXXIX. PART II. (NO. 11). 3 F 



