ana their allied Compounds. 271 



compounds; these latter being the sesquioxides generally, and 

 the so-called metallic acids. This union has not been adopted 

 from any preconceived views, but has been forced upon me in 

 the course of these investigations. The minerals of the above 

 series are, in fact, so linked together, that it is frequently impos- 

 sible to separate them without breaking through self-evident 

 analogies. I am willing to allow that, in places, I may have 

 gone too far j but in a system like that employed, much is ne- 

 cessarily left to tbe judgement ; and if the entire subject be care- 

 fully reviewed, I think it will be admitted that tbe present essay 

 has upon the whole an opposite tendency. Several of the pro- 

 posed types, for instance, might upon certain grounds have been 

 united ; but they have been kept separate, partly in deference 

 perhaps to almost irradicable prejudices, which bias one's opinion 

 even against the will, and partly as a matter of convenience. It 

 is to be remarked, however, that the present arrangement is 

 merely offered as a provisional one ; and that, from the difficul- 

 ties with which the subject is beset, it is easier to point out im- 

 perfections than to remedy them. 



Minerals of the Silica, Alumina, and Metallic Acid 

 Series. 



Braunite Type ; — Dimetric. 

 Braunite, Mn 2 O 3 . 



Corundum Type : — Hexagonal. 



1. Corundum sub-type. — Erysiderite, Fe 2 3 (Appendix, hy- 

 drosiderite, Fe 2 3 + xH 2 0) ; ilmenite (Fe 2 3 , Ti 2 3 ); corundum, 

 A1 2 3 (App. hydrargillite). 



2. Quartz sub-type. — Quartz, Si 3 (App. opal); phenakite, 

 Be 2 3 , SiO 3 ; beryl (Be 2 3 , A1 2 3 ), 2Si0 3 . 



Staurolite Type : — Trimetric. 

 Staurolite (Al 2 O 3 , SiO 3 ) ; andalusite; chrysoberyl (Be 2 3 , 

 Al 2 O 3 ) ; topaz {(Al 2 3 , SiO 3 ) + (A1 2 F1 3 , SiFl 3 )}. 



Cy unite Type : — Triclinic, Monoclinic. 

 Pycnite (?) ; cyanitc ; wserthite ; sillimanite ; monrolite (?) ; 

 bamlite ; euclasc, 4(Be 2 3 , A1 2 3 ), 3Si0 3 . 



Spinel Type .- — Monomelric. 

 General formula =RO, R 2 3 . Spinel; pleonaste; gahnitc; 

 franklinitc ; cliromolite ; zophosine (pitchblende) ; magnetite, 

 FeO, Fc-O 3 ; iscrine; garnet, including pyrope, &c. ; hclvine*; 

 eulytine (?). 



In order to <i:conomisi: space, J haw not cited the different 

 * See the present volume of the Philosophical M;i<r;i/.inc, Art. XXI. 



