VIEWS ON MINERAL SPECIES. 445 



A great deal dates from the discovery that the plagio- 

 clastic Felspars must be regarded as isomorphous mixtures 

 of the two compounds Na Al Si 3 O s and Ca Al 2 Si 2 O s , a 

 relation which later investigation has established more 

 firmly than ever. In whatever way such a relation is 

 explained, whether by regarding Ca as capable of replacing 

 Na, and Al of replacing Si, or by supposing Ca AL to be 

 capable of replacing Si 2 , the exigencies of this and similar 

 cases lead far beyond the old conceptions of isomorphism, 

 and introduce radicles which result in quite a new mode of 

 chemical classification. 



The replacement of Fluorine by hydroxyl, for example, 

 enables us to refer many so-called hydrated compounds to 

 a natural position among anhydrous groups. Thus 

 Swabite, a newly discovered hexagonal mineral having 

 the composition H 2 0. ioCaO. 3As 2 5 , can in the form 

 (CaOH) Ca 4 (As0 4 ) 3 be reasonably referred to the 

 Apatite group (Ca F) Ca 4 (P0 4 ) 3 , to which it bears much 

 resemblance. 



These principles are being extended every day. 

 Brogger and Backstrom have proposed to include in the 

 Garnet group the minerals Sodalite, Nosean, and Lapis- 

 Lazuli, which were formerly either allied with Nepheline as 

 alkaline silicates, or were placed in separate groups as 

 silicates combined with chlorides and sulphates. They 

 suo-oest, in order to bring them into harmony with the 

 Garnet type R 3 Al 2 (Si 4 ) 3 , the following formulae :— 



Sodalite Na 4 [Al CI] Al 2 (Si 4 ) 3 . 



Nosean Na 4 [Al (Na S0 4 )] AL (Si 4 ) 3 . 



Lapis- Lazuli Na 4 [Al (S 3 Na)] AL (Si 4 ) 3 . 



Again, according to Brogger, another group of minerals 

 which present remarkable resemblances in crystallographic 

 and physical characters include certain silicates, aluminates, 

 ferrates, and manganates conforming to a general type of 

 formula R 2 Si 4 and R 2 X 2 4 , where in the silicates R 

 may be any of the elements Ca, Fe, Mg, Mn, Zn, Be, or 

 the radicle (H 2 Cu), while in the aluminates and other com- 

 pounds it may denote Be, H 2 , or Mg 2 O ; X being 

 Al, B, Fe, or Mn. This unites into one mineral group 



