1885.] Mineralogy and Petrography. 705 
MINERALOGY AND PETROGRAPHY.! 
THE CLASSIFICATION OF NATURAL SILICATES,2—The author dis- 
cussed the history of mineralogy, and noticed the method of 
11SiO,, and pyroxene to one with 14SiO,, or some simple multi- 
ple of these numbers. In such compounds the degree of com- 
cate the mean equivalent weight of its atomic unit, corresponding 
to an atom of NaCl, for which purpose H,O and CaO are divided 
mal composition), is (ca.al2.si3) 06; the small letters representing 
atoms, ando= 8. This gives an equivalent of 107, which di- 
vided by six yields for the mean atomic weight of the unit in 
both of these species P = 17,83. Dividing this latter number by 
2.7, as the specific gravity of meionite, we have for the atomic 
volume in this species V = 6.60, and by 3.4, as the specific grav- 
ity of zoisite, V = 5.24. The true formulas and equivalent weights 
of these polysilicates must be deduced from a comparison of their 
specific gravities with those of species whose equivalent weights 
are otherwise determined. Meanwhile it will be seen that zoisite, 
with the lower value of V or, in other words, the more condensed 
‘Edited by Dr. Geo, H. WILLIAMS, of the Johns Hopkins University, Baltimore. 
? Abstract of a paper read before the Nat. Acad. Sciences, April 21, 1885. 
