328 Revieivs — Diqmrc 8^ Mrazec — Iron Ore of Troitsk. 



pegmatites ; {d) garewaites (granular aggregates of olivine, basic 

 plagioclase, and chrome-iron, with porphyritic crystals of pyi'oxene). 

 Kocks composed largely of the last mineral, which frequently belongs 

 to the diopside division (or occasionally hornblende), seem to be very 

 characteristic of the northern part of the Ural chain. The former 

 memoir distinguished one of these, composed of lamellar pyroxene, 

 with olivine, magnetite, and some hornblende, by the name koswite, 

 and the present one describes and figures other examples, together 

 with tilaite, a gi-anular rock, consisting of pyroxene (chiefly), with 

 magnetite or spinel, a little plagioclase felspar, and occasional 

 representatives of the following : olivine, hypersthene, hornblende, 

 and biotite. The rock evidently is intermediate between the true 

 pyroxenites and the gabbros, to one or other of which it may incline. 

 Chemical analysis shows it to be richer in alumina, lime, and silica, 

 but poorer in magnesia than the picrites, while it is richer in lime and 

 silica, and poorer in alumina, lime, and magnesia than the troktolites. 

 Thus the coinage of a new name is justifiable. The rocks of the 

 Northern Urals evidently contain some very interesting types, and 

 Professors Duparc and Pearce deserve the gratitude of students for 

 their exhaustive memoirs. T. G. B. 



II. — Le Minerai de Fer de Troitsk. By Professor L. Duparc 

 and Professor L. Mrazec. pp. llo, 6 plates, 2 geological 

 maps, and 13 figures in text. (St. Petersburg, 1904.) 



I')ATHER thick banks of magnetite are worked, at more than one 

 \ place, near the village of Troitsk, in the Urals, at the junction 

 of a granite-porphyry and a black schistose rock, referred to the 

 Lower Devonian. M. Krasnopolsky gave a short description of the 

 district in 1889. He and Professor Mrazec now publish a more 

 minute account, accompanied by full petrographical and mineral 

 details, proposing an hypothesis to explain the presence of the ore 

 that is found to occur as enclosures in the granite-porphyry as well 

 as in layei'S which are separated by bands of ' corneenne ' variable in 

 thickness. Of the latter there is more than one type, of which they 

 give petrographical studies, assigning its metamorphism to the granitic 

 rock, which also is fully noticed. The Devonian strata are shown to 

 be considerably more recent than the ' corneenne.' As the authors 

 agree with Professors Michel-Levy, Barrois, and La Croix in attri- 

 buting important chemical changes to the action of intrusive 

 rocks like granites (admitting, however, that there are two sides to 

 the shield), they offer the following explanation of the presence 

 of the magnetite. This was introduced into the 'corneeniies ' by 

 the magma which furnished the granite-porphyry, but its position is 

 due to the action on the sedimentaries, anel comes from a fixation in 

 them of the iron, derived from that magma. They believe the latter 

 to have mounted slowly, and to have formed, under orogenetio 

 ]>ressures, a kind of laccolite which absorbed some of the covering 

 rocks and developed the magnetite in adjacent masses, thus forming 

 the zones of ore near the contact. Then increased pressure ruptured 

 the covering rocks, ejecting a quantity of magma, and enveloping 



