368 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. 8. 
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- z 
panying figure, illustrating these principles, represents a macle of 
Staurotide, discovered by Dr. C. T. Jackson, resembling those of 
