SALMON — ON ROCKS. 



229 



a marked series, at one pole of which are the silicates of the alcaline 

 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 liornhlendes and augites. 



Felspar group. — The felspars are essentially silicates (of various 

 orders) of Al and the R 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 veiy 

 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 11 

 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 ortlioclase, 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 defectueuse. En effet, les especes 

 qui la constituent n'appartiennent pas au meme systeme crystallin ; la compo- 

 sition, differente pour quelques-uns sous le rapport des elements, est atomique- 

 ment difterente pom' la plupart : en sorte qu'il n'existe de rapprochement entre 

 eux ni par la forme ni par la composition. Les caracteres exterieurs sont, il est 

 \Tai, tellement analogues, que la reconnaissance de ces especes est une des plus 

 grandes difficultes de la mineralogie. En outre, les formes quoique difFerentes 

 sous le rapports des systemes auquels elles appartiennent sont tres-rapprochees par 

 leurs angles. La coulem, T eclat, la durete, le poids specifique, presque les memes 

 pour ces mineraux, augmentent leur analogic : leur reunion en un gi-oup est done 

 fondee plutdt sur la difficulte qu'on eprouve a les reconnaitre que sur les principes 

 philosophiques. Coqucmd, p. 4. 



