in Spain and Russia. 177 



In consequence of this discovery the two authors of the memoir 

 under review, one of whom, M. Duparc, has made a special study of 

 the Russian deposits, visited the Spanish locality in order to compare 

 the modes of occurrence in the two countries. Their work was 

 greatly facilitated by M. Orueta, who supplied them with a copy of 

 his unpublished map. 



The memoir begins with a description of the topography, geology, 

 and petrography of that portion of the Sierra de Ronda in which 

 the platiniferous deposits occur. The principal constituents of the 

 ultrabasic rocks are spinels, rhombic and monoclinic pyroxenes, and 

 olivine. These constituents are mixed in varying proportions. The 

 most common variety is composed of brown spinel, olivine, and 

 a rhombic pyroxene (hartzburgite). Next to this comes a variety 

 containing a monoclinic pyroxene in addition to the above-mentioned 

 constituents (lherzolite). A third type corresponds to dunite. But 

 all these varieties shade into each other and all are liable to 

 serpentinization. The authors compare the structure of those areas 

 in which dunite is found to a sponge ; dunite filling the hollows and 

 the other varieties forming the network. They consider that the 

 peridotites were intruded into the surrounding gneissose and more 

 or less metamorphosed sedimentary rocks after the Cambrian period 

 and before the formation of a conglomerate, which is either of 

 Permian or Triassic age ; but no satisfactory evidence of the reference 

 of any of the surrounding sedimentary rocks to the Cambrian period 

 is given. 



M. Orueta supplied the authors with a few small grains of platinum. 

 Two varieties were noticed. One shows under the microscope 

 polygonal depressions which are precisely similar to those observed 

 on grains derived from pyroxenic rocks in the Urals. They possess 

 a yellowish bronzy lustre and have generally been much rounded. 

 The other variety, also rounded, appears absolutely black. Heated 

 in a borax bead the patina disappears, and the borax on cooling- 

 assumes a faint greenish tint. The platinum acquires a silvery 

 metallic appearance. The removal of the patina discloses the presence 

 of minute hollows filled with a black mineral which the authors 

 believe to be chromite. Precisely similar grains are found associated 

 with chromite in the Urals. Microchemical tests revealed the 

 presence of osmium, platinum, palladium, copper, iron, and nickel. 

 The largest specimen examined weighed -02338 gram ; butM. Orueta 

 has obtained much larger ones and also a small nugget weighing 

 2 grams. 



The authors then describe two or three of the typical Russian 

 occurrences, laying special emphasis on the differences between the 

 two localities. In the Urals dunite is developed on a much more 

 extensive scale. It forms large more or less elliptical masses 

 surrounded by a fringe of pyroxenic rocks which are, in their turn, 

 surrounded by gabbro. In the Sierra de Ronda the dominant 

 peridotite contains a rhombic pyroxene, and the mass is directly 

 surrounded by metamorphic rocks into which the peridotites have 

 been intruded. Dunites occur in both localities, but in the Urals 

 they are, as a rule, sharply separated from the pyroxenites, whereas 



DECADE VI. — VOL. IV. — NO. IV. 12 



