SALMON — ON ROOKS. 229 
a marked series, at one pole of whicli are the silicates of the alcaliuo 
bases (K, Na, Li), — and at the other pole the silicates of the bases 
Mg, Mn, and Fe ; the bases Al and Ca being in a certain sense inter- 
mediate. If the simple alcaline silicates existed in nature (which, as 
stated, is not the case by reason of their extreme solubility), they 
would occupy the one pole, now occupied by the compound silicates 
of the alcalies with A (felspars) ; — with talc and olivine (silicates of 
Mg and Fe) at the other pole. Between these extremes we may 
place, next the felspars, the garnet group ; then the micas ; and, next 
the talc group, the hornblendes and augites. 
Felspar group. — The felspars are essentially silicates (of various 
orders) of Al and the E. bases, K, Na, Li, and Ca, having the constant 
oxygen ratio between the Al and the R bases of 1 : 3. The order of 
these silicates, and the consequent proportion of acid, varies very 
widely; in Petalite, containing the most Si, the O.Q = -25; while 
in Anorthite, containing the least Si, it is as high as 1.0. Thus the 
felspars differ among themselves in the proportion of Si they contain, 
and in the replacement of the alcaline bases and Ca for each other ; 
the only constant relation being the oxygen ratio of the Al and the K 
bases. The most recent analyses show that all felspars contain both 
K and Na, but the proportions are very variable, the Na almost dis- 
appearing in some, as orthoclase, and the K in others, as albite. The 
Ca felspars seem to be the source of a class of hydrous silicates called 
zeolites. The species of the felspar group have a very striking physical 
similarity, unmistakeable in practice, but not very readily defined.* 
Garnet and Tourmaline group. — This group, in the list of minerals, 
extends from Andalusite to Cordierite. It is characterised by the 
absence, or only trifling quantities, of alcalies in its species. 
* L'expression de famille de felspaths est defectueiise. En efFet, les especes 
qui la constituent n'appartiennent pas au menie systeme crystallin ; la compo- 
sitiun, ditterente pour quelques-uns sous le rajiport des elements, est atomique- 
nient ditf'ereute pour la plupart : en sorte qu'il n'existe de rapprochement entre 
eux ni par la forme ni par la composition. Les caracteres extcrieurs sont, il est 
vrai, tellement analogues, que la reconnaissance de ces especes est une des plus 
grandes difficultes de la mineralogie. En outre, les formes quoique dlft'erentes 
sons le rapports des systemes auquels elles appartiennent sont tres-rapprochees par 
leurs angles. La couleiu-, I'eclat, la durete, le poids specific[ue, presque les memes 
pour ces mineraux, augmentent Icur analogie : leur reunion en un group est done 
londce plutot sur la ditlicultc qu'on eprouve a les reconnaitre que sur les priiicipes 
phUosoplxiques. Coqiumd, p. 4. 
