36S Review of Dana's Mineralogy. 



the existence of the mineral as a normal chemical salt, will 

 prove to be only mechanical mixture. Indeed, already we dis- 

 cover with pleasure a disposition in foreign chemists of emi- 

 nence, to simplify as far as possible their formulas. Mr. Dana 

 has in his preface the following judicious remarks on this subject. 



" Notwithstanding the well-known principle that crystallizing sub- 

 stances may include, mechanically, the impurities present in a solution, 

 a fact often discoverable with the naked eye, chemists very generally in- 

 clude in the formula every ingredient obtained by analysis, however small 

 the proportion. In some species, as quartz, lime, heavy spar, celestine, 

 macles of andalusite, auriferous pyrites, and a few others, mechanical 

 mixtures are allowed ; but in most cases, especially if the mineral be a 

 complex one, mechanical impurity seems hardly to be thought of as a 

 possibility : while, in truth, the detection of an ingredient, in small 

 quantity, in an opaque crystallized mineral, is neither proof of its me- 

 chanical, nor of its chemical combination ; and some farther evidence 

 should be required before coming to any conclusion on this point. Had 

 the possibility of mechanical mixtures been more considered, and a 

 doubt indulged when chemistry seemed to clash with crystallography, 

 the science would have been encumbered with fewer synonyms. As 

 an example : — the Peristerite of a British chemist would have been 

 left in undisturbed union with feldspar : it requires but a common mag- 

 nifier to detect the impurities (minute spangles, apparently of mica) in 

 the red stripes of this red-and-white iridescent feldspar from Upper 

 Canada ; and it is very probable that quartz may be segregated, on 

 known principles, in the white stripes, like the mica in the red. These 

 facts explain the peculiar composition of this mineral, the analysis of 

 which Rammelsberg quotes with expressions of distrust ; and if their 

 bearing on the composition of other minerals were admitted, we should 

 find the chemist less hasty in urging forward new species on chemical 

 grounds alone." 



The impurities often take a symmetrical arrangement, gen- 

 erally collecting most abundantly about the centre and along the 

 diagonal, and also in planes between the centre and edges of the 

 crystal. In chiastolite the foreign matter Fig. 7. 



is arranged about the central axis, and in 

 planes running from this axis to the edges, 

 and also about the lateral edges and ex- 

 terior surfaces of the crystal. The accom- 

 panying figure, illustrating these principles, represents a made of 

 Staurotide, discovered by Dr. C. T. Jackson, resembling those of 



