38 Reviews — Bohemian Basalts and Phonolites» 



tlie minerals produced under sucli circumstances. And this is fol- 

 lowed by a discussion of the nature of the secondary minerals formed 

 in the cavities of the diiferent basalts during their decomposition, 

 and of the relations which these bear to the several original con- 

 stituents of the rocks. 



In his monograph on the Bohemian phonolites, the author follows 

 the same general plan as in that on the basalts ; we cannot, however, 

 do more than notice its more salient conclusions. 



The microscopical and chemical characters of the fifteen species of 

 minerals found in the phonolites are in the first place discussed. 

 These minerals are as follows : sanidine, triclinic-felspar, leucite, 

 nepheline, nosean, haiiyn, tridymite, hornblende, augite, magnetite, 

 titanite, pyrite, spinell, mica, and apatite. 



The variations in the relative proportions of the two principal 

 constituents in those phonolites give rise to a regular series of rocks, 

 ranging from the neplieJine-plionolites, with from 45 to 37J per cent, 

 of nepheline and 43 to 50 of sanidine, to the sanidine-pJionolites, 

 with from 9 to 17 per cent, of the former mineral, and 58 to 71 per 

 cent, of the latter. Between these extremes range numerous 

 varieties, distinguished by the presence, in greater or less abundance, 

 of leucite, nosean, haiiyn, and oligoelase. Of such principal varie- 

 ties Dr. Boritsky recognizes no less than eight, and he shows that an 

 approximation to their mineral constitution may be arrived at from 

 an examination of their specific gravities, their behaviour with acids, 

 and their sulphur reactions. As in the case of the basalts, the nature 

 of the metamorphic changes produced in the rocks through which 

 the phonolites are extruded is very fully discussed, and also the 

 nature of the minerals formed in their cavities during decomposition. 



The limits of this magazine notice will not admit of our entering 

 into any further details on the subjects treated in these valuable 

 monographs ; but we may add that the coloured lithographic plates 

 which accompany them rank with the very best illustrations of the 

 microscopic structure of rocks that have yet been published. 



We need scarcely add any remark of our own upon the importance 

 to the geologist of such detailed works on local petrology as that we 

 have been noticing. When we possess a number of such contribu- 

 tions to the history of different volcanic districts, many important 

 problems — such as those of the relative characters of volcanic 

 materials in different geological periods, and in different areas 

 during the same period — will obtain a satisfactory solution ; while 

 questions now relegated to the domain of speculation — such as that 

 of the origin of volcanic outbursts from a central incandescent 

 nucleus or from a number of more or less independent sources — 

 will be brought within the sphere of inductive research. 



We understand that Dr. Boritsky purposes to follow up these 

 monographs on the Tertiary volcanic rocks, by a series of other 

 memoirs on the petrology of Bohemia ; and we are assured that 

 alike in England, Germany, and France, where the microscopical 

 study of rocks is now being pursued by so many earnest investigators, 

 the appearance of these will be looked forward to with the greatest 

 interest. J. W. J. 



