72 THOMAS STERRY HUNT ON A NATURAL SYSTEM IN 



pounds may be directly formed. That uephelite, for example, may, as is supposed, under 

 favoring eircumstauces, be transformed into pinite, is due to the chemical stability under 

 these circumstances of this latter compound, and the frequent occurrences of pinite and 

 the similar silicates, under such conditions, are illustrations of the survival of the littest 

 in the inorganic world. But these very relations which conduce to the production of such 

 stable compounds by epigenesis, that is to say as a result of transformation through 

 aqueous action, are evidently such as would lead, under favouring conditions, to the direct 

 generation of the same or chemically similar compounds. Thus, for example, while the 

 pseudocrystalliue form of pinite in the case just mentioned shows its epigenic origin, the 

 occurrence in granitic A'einstones, in intimate association with orthoclase, tourmaline and 

 quartz, of muscovite, a silicate very similar in centesimal composition to pinite, is 

 unquestionably an example of direct production from solution. As regards the large 

 beds of stratiform rocks evidently of acjueous origin, made up chiefly or entirely of 

 a silicate ha^âng the composition a)id the physical characters of pinite, such as occur 

 at various geological horizons, and, as I have elsewhere described, ' it seems not less 

 diHicult to assign to them an epigenic origin. In other words, the beds of pinite, or 

 beds of mica-schist, like the mviscovitic micas of granitic veins, have not been produced 

 by the transformation in silu of more highly alkaliferous silicates, but are either the 

 results of the subsequent molecular rearrangement or diagenesis of sediments deriA^ed 

 from partially decomposed silicates, perhaps not without admixtures of aqueous deposits 

 of chemical origin, or else, as in the case of such minerals in veinstones, are entirely from 

 the latter source. 



§ 94. Crystalline forms, as displayed in what are called pseixdomorphs, may and often 

 do give evidence of transformations through aqtieous agency, in silicates, as in other orders 

 of minerals. Such changes haA'e been especially effected in fissures and open cavities, 

 which have been channels for waters of changing composition and temperatiire, during the 

 long process which has filled these openings with mineral masses. In this way, crystals 

 deposited at one stage are attacked at another, and are either more or less completely dis- 

 solved or transformed into insoluble products which are now found surrounding nuclei of 

 the unchanged mineral, or in some cases penetrating its substance. Examples of such 

 actions are familiar to all who have studied attentively the history of granitic and related 

 veinstones. Care should, however, always be taken in the study of pseudomorphs to keep 

 in mind another and a different phenomenon, namely, that resulting from the power of a 

 substance in the process of crystallization to cause other bodies to assume its own 

 geometric form. Examples of these are seen in the cases of calcite, dolomite and gypsum 

 crystallizing in the midst of siliceous sand, by which are generated such aggregates as the 

 so-called crystallized sandstone of Fontainebleau, which, while liaving a crystalline shape 

 belonging to calcite, includes from 50 to 63 per cent, of quartz grains. A not less remark- 

 able case is seen in staurolite, which, according to Lechartier, retains its crystalline form 

 and general aspect, even when. by the inclusion of foreign matters, chiefly quartz, the pro- 

 portion of silica is raised from the normal content of 28.0 to 50.0, and even 54.0 i^er cent., 

 corresponding to more than one part of quartz with two parts of staurolite, the mixture still 

 retaining the crystalline form of the latter species. Thus a compound in crystallizing may 



' Trail.?. Roy. Soc. Canada, Vul. ii. Sec. o. p. 52, and Geology ol' Canada, ISOu, p. -ISi-lSG. 



