Bohemian Basalts and Phonoliies. 37 



scriptions that Dr. Boritsky's researclies, thongli conducted quite 

 independently of those of vom Rath in the Southern Tyrol, and of 

 Szabo in Hungary, seem to lead to precisely the same conclusion as 

 theirs, — namely, that but little importance can be attached to the 

 supposed necessary association of the several felspars with, different 

 classes of rocks. 



VI. Tachylite-hasalt. — When the basis of a trachy-basalt assumes 

 a glassy texture, the variety to which Dr. Boritsky applies the above 

 name is constituted; It forms, however, only veins of a few inches in 

 width, or the sides of dykes of larger size composed of trachy-basalt. 



It might be anticipated from the peculiar mineralogical constitu- 

 tion of these Bohemian basalts that they would, in their chemical 

 composition, present some differences from that of the normal rocks 

 of the same class ; and that such is really the case is proved hj the 

 numerous analyses, made by himself and other chemists, which Dr. 

 Boritsky discusses in the third part of his monograph.. 



In the magma-, nepheline-, and leucite-basalts the per-centage of 

 silica is exceedingly low, varying between M and 43 per cent, only ; 

 while the proportion of alkalies is high, — ^namely, from 5 to 10 per 

 cent. Our space will not permit us to enter upon the consideration 

 of the actual quantitative determination of the mineralogical consti- 

 tution of the various rocks, — for which, however, the author supplies 

 ai^proximate estimates, based on a discussion of their oxygen ratios, etc. 



In the succeeding part of his monograph. Dr. Boritsky proceeds 

 to show that the several varieties of basalt were erupted during dif- 

 ferent stages of the great series of volcanic outbursts in Northern 

 Bohemia, which, commencing with the Oligocene period, continued 

 almost down to recent times. 



The magma-, nepheline-, and leucite-basalts belong to the earliest 

 period, and form the great lava-streams and tuffs which constitute 

 the plateaux occupying so large an extent of country. These were 

 succeeded by the different varieties of felspar-basalts, veins and in- 

 trusive masses of which are found penetrating them in all directions. 

 Then came the period of the sporadic phonolitic outbursts, which 

 Dr. Boritsky justly recognizes as rocks extruded in a viscous or 

 semi-solid condition. And lastly the trachy- and tachylite-basalts — 

 veins of which traverse the phonolites and other older masses — made 

 their appearance. 



The author is further of opinion that a relation can be detected as 

 existing between the 'prevalent directions of the dykes of basalt and 

 their petrological character and age. Perhaps, however, further 

 evidence than is adduced in this memoir may be required, before 

 geologists will regard this conclusion as fully established. 



Of the concluding portions of the monograph, we can do no more 

 than mention the subjects which they treat, — only premising that 

 these are handled with the same thoroughness and ability which 

 characterize the earlier parts of the works. In the first place, we 

 have an examination of the interesting effects of local metamorphism , 

 produced on the Cretaceous and other rocks, by the intrusion of the 

 different varieties of basaltic rock among them ; and of the nature of 



