36 Reviews — Prof. Boritshy — 



different mineral species which are found in these rocks, and of the 

 means by which they may be best identified under the microscope. 

 No less than twenty-seven species or distinct varieties of minerals 

 have been detected in these basalts, — namely augite, diopside, horn- 

 blende, diallage, bronzite, enstatite, hypersthene, biotite, rubellane, 

 oligoclase, andesine, anorthite, labradorite (?), nepheline, leucite, 

 haiij^n, nosean, garnet, melilite, olivine, apatite, magnetite, hsematite, 

 titanite, j)icotite, pyrrhotine, and tachylite. Where, as in not a few 

 instances, microscopic examination alone is inadequate for the deter- 

 mination of a constituent of the rock, the fact is candidly admitted 

 by the author, and chemical or other methods resorted to. 



As the result of these studies, it appears that the Bohemian basalts 

 may be divided into six classes, which Dr. Boritsky defines as 

 follows : — 



I. Magma-basalts, consisting of a glassy basis, through which 

 cr3^stals of augite, olivine, and magnetite are scattered. The fel- 

 spathic ingredient of the rocks has not crystallized at all, but 

 remains in the condition of a glass exhibiting numerous microliths. 

 This group of basalts is further subdivided by the author into a 

 number of subordinate varieties, based on the colour of the glassy 

 basis and the size of the crystals of the embedded minerals. 



II. Nepheline-hasalts. — In these the magma, in which crj'^stals of 

 the same minerals as in the last group are embedded, shows 

 indications of breaking up into the crystalline forms of nepheline 

 (neiilielinitoid hasalt), or finally passes into a distinct aggregate of 

 nepheline crystals (nej^helinite) . This rock, like the last, exhibits 

 many varieties of texture, and may also contain greater or less 

 proportions of nosean or hauyn ; and on these grounds it has been 

 divided into a number of subordinate varieties. 



III. Leucite- Basalts. — This rock shows, in a manner similar to 

 the last, the gradual passage of a glassy magma into leucite and 

 nej)heline, and is divided, according to the more or less complete 

 cr3'stallization of its basis, into leucitoid hasalt, pepenno-hasalt and 

 leiicitopJiyre ; while other varieties are based on the size of the 

 included crystals. 



IV. Felspar-hasalts. — These fall into three series : 



1. Melaphy re-has alt, in which the basis remains to a great 

 extent in a glassy condition. 



2. Common felspar -has alt, in which the felspathic ingredient 

 is distinctly crystallized out; but this proves, according to Dr. 

 Boritsky, to be oligoclase or anorthite, and seldom or never 

 labradorite ! 



3. Andesite and Plionolite-hasalts, in which the basis consists of 

 oligoclase felspar, with nepheline, leucite, nosean, etc. 



V. TracJiy -basalt. — In this group of basalts olivine never occurs, 

 but the mass is made up of crystals of orthoclase and oligoclase 

 felspar, and of nepheline and nosean or haiiyn, among which are 

 scattered others of biotite and hornblende, and more rarely of augite. 

 The proportion of silica in these remarkable rocks only reaches 

 an average of 45 per cent. It will be seen from the above de- 



