BISCHOP CHEMICAL GEOLOGY. 9 



n vegetable and animal structures, as well as in sedimentary rocks, 

 with remarks on the continued circulation of phosphoric acid from 

 the minerals, through plants to animals, and its return through the 

 decomposition of organic remains to the mineral state, close the 

 chapter. 



Boracic acid and borates form the subject of chapter xxiii. ; the 

 occurrence of boracic acid in the Suffioni of Tuscany associated with 

 sulphate of ammonia and chloride of ammonium is described ; and the 

 decomposition of boracite in the limestone from which the hot springs 

 rise is suggested as a means of origin. The formation of tourmaline, 

 &c., is reserved for the chapter on the origin of gi.^anite. 



The third and most important part of the work, comprising chapters 

 xxiv. to xli., extending to nearly 600 pages, treats of the various siK- 

 cates found in nature, and the general remarks at the commencement 

 treat of their probable composition and origin. Of the former sub- 

 ject it is remarked that the formulae in use for the more complex 

 silicates are of little other value than that of expressing the indivi- 

 dual views of the chemists by whom they are propounded. Through- 

 outthis portion of the work the composition of the minerals is expressed 

 chiefly by their oxygen quotients, that is the fractions obtained by 

 dividing the amount of oxygen in the protoxide and sesquioxide 

 bases taken together by that of the silica. "With regard to the origin 

 of crystalline sihcates, the author is disposed to consider them as 

 essentially formed by the action of water from amorphous masses ; 

 thus he remarks that, as a rule, lavas, especially those of newer date, 

 are not crystalline, although when sufficiently thick they may take 

 years in cooling ; this view is especially brought out in the conside- 

 ration of the origin of leucite, a mineral which occurs most abundantly 

 and in the largest crystals in Yesuvian lavas of pre-historic origin. 

 Even at ordinary temperatures glass becomes crystalline when ex- 

 posed to atmospheric changes for a long period, as has been shown by 

 Brewster and Zirkel in mediaeval glass taken from the windows of 

 Saint Andrew's and Cologne Cathedrals. 



The system of arrangement of the descriptive notices of the sili- 

 cates is based upon the relations of their pseudomorphs to each other, 

 the series being commenced with the zeolites, which never appear 

 except in their own proper forms ; felspar is made to take the second 

 place, from the fact that orthoclase has been found in the forms of 

 laumonite and analcime, and therefore may under certain circum- 

 stances be derivable from zeohtes. The last members of the series 

 are the difficultly decomposable substances : mica, chlorite, steatite, 

 serpentine, &c., which are found as pseudomorphs of almost all the 

 harder silicates. Especial reference is made to the remarkable con- 

 trast between the extreme divisibility of mica due to physical structure 

 and its almost absolute stabihty against chemical changes from the 

 action of the air. This property is assumed to be due to peculiar 

 isomeric conditions of the component silicates. As might be supposed 

 in a substance forming the ultimate product of alteration of a great 

 number of different minerals, micas vary very much in composition, 

 the maxima and minima of the chief components are given as fol- 

 lows : — 



VOL. XXII. PART II. C 



