796 General Notes, [August, 
molecule, differs from the less condensed meionite by its greater 
hardness and its superior resistance to the action of acids. 
rom a consideration of the constitution thus assigned to sili- 
cates, it follows that the comparatively simple ratios generally 
deduced for the silica and the various bases are, in many cases, 
but approximations to the more complex ratios really existing. 
These, from the frequent impurities of natural silicates, can sel- 
dom be fixed with exactness, though with sufficient precision to 
give very nearly the values of P and V, which latter serves to 
determine the place of the species in the natural system of classi- 
fication. 
Water being an element universally distributed, its presence or 
absence in a silicate becomes of subordinate importance in deter- 
mining alike the genesis and the natural affinities of species, so 
that the water-ratios are omitted in the tables of classification 
(which were shown), wherein the various natural silicates are, from 
the chemical side, considered with regard to the atomic ratios ot 
fixed bases to each other and to the silica. There are genetic 
reasons for separating silicates of sesquioxyd bases, like alumina 
from protoxyd-silicates. The former of these constitute the class of 
Persilicates, and the latter the Protosilicates ; those containing 
both protoxyds and sesquioxyds being designated Protopersili- 
cates. Ferric oxyd and zirconia are classed with alumina as ses- 
quioxyds, while titanic and boric oxyds in silicates are counted 
with the silica in determining the atomic ratios. 
- In the table of the Protosilicates, and in that of the Persilicates, 
both hydrous and anhydrous, the generally accepted ratios of the 
fixed bases to the silica are noted, but in that of the Protoper- 
silicates regard is had to the more important ratios of sesquioxyds 
d fixed peroxyds to each other, inasmuch as the ratio of silica 
to both of these is found to vary greatly in closely related spe- 
cies, as may be seen in zeolites, feldspars, scapolites and micas. 
In these tables the three great classes of silicates are each 
arranged in groups, with primary reference to physical characters. 
_ Thus for Protosilicates we have in parallel columns, Pectolitoid, 
Spathoid, Adamantoid and Ophitoid, with each of which the 
range of values for V is given, while in an adjacent column to 
the left are inscribed the approximate atomic ratios of fixed pro- 
toxyds to silica. Among pectolitoids are included with pectolite, 
apophyllite and datolite, hydrohodonite, pyrosmalite, dioptase, 
_ calamine, cerite, thorite, etc. The spathoids embrace willemite, 
tephroite, gadolinite, helvite, leucophanite, wollastonite and tscheff- 
kinite ; the adamantoids, chondrodite, chrysolite, phenacite, bert- 
Yandite, hornblende, pyroxene, titanite, and danburite, while the 
oids include various hydrous silicates, of which villarsite, 
Serpentine and talc are representatives, 
__ The Protopersilicates, in like manner, are grouped under the 
s of Zeolitoid, Spathoid, Adamantoid and Phylloid, a subor- 
