1 87 1.] Mineralogy. 285 



tributaries yield their quota ; and as in their basins no clearing of consequence 

 has as yet taken place, their floods have not become much earlier in date, and 

 the highest level is only reached at Astrachan three days earlier than was 

 formerly the case. The duration of the floods, which in the last 10 years is 

 nearly double what it was at first, is probably to be accounted for by the fact 

 that clearing of woods promotes more rapid drainage, and so the floods 

 in the rivers and the desiccation of the soil proceed pari passu. The con- 

 tinuance of the flood is therefore due, not to the melting of the snow, but to 

 the rainfall, which was formerly retained on the land and is now discharged 

 more rapidly. 



In connection with these changes it is very interesting to learn that the 

 level of the Caspian has risen very materially of late years, as is proved by the 

 soundings in the port of Baku. M. Wojeikoff promises to return to the subject 

 at a future date. 



The journal also contains full descriptions of the auroras of October 24 and 25, 

 from several European Stations. 



MINERALOGY. 



Although it is only within the last few years that attention has been forcibly 

 directed to the importance of studying the minute structure of minerals and 

 rocks, sufficient work has nevertheless been already accomplished to encourage 

 the belief that the microscope will eventually become almost as important an 

 aid to the mineralogist and petrographer as it has long been to their biological 

 brethren. Among the most successful workers in the department of micro- 

 minerology may be mentioned Professor Zirkel, of Kiel, whose recently 

 published researches* it becomes our duty to notice. 



Whilst it is well known that crystals of quartz frequently contain many 

 microscopic cavities charged with certain fluids, such enclosures are but rarely 

 found in crystals of felspar. It is, therefore, not without interest that Zirkel 

 announces the discovery of a very large number of fluid cavities in the felspar 

 of a coarsely crystalline rock from the Isle of Mull, consisting of plagioclase, 

 diallage, and olivine. It is found, too, that the crystals of labrador-felspar 

 from the olivine-gabbro of the Isle of Skye are rich in similar fluid cavities. 

 In many of the liquid-bearing hollows with which quartz is frequently charged, 

 may be detected minute cubic crystals, usually clear and smooth faced, but 

 sometimes having their planes striated in squares. Zirkel finds that many of 

 these cubes consist of sodium-chloride, and believes that the fluid which 

 encloses the crystals is a solution of the same salt — results which corroborate 

 the conclusions advanced many years ago by Mr. H. C. Sorby. 



Attention is directed by the same author to the wide distribution of 

 microscopic crystals of apatite, and a long list is published of many eruptive 

 rocks — diorites, melaphyres, and diabase — in which this mineral may be 

 detected. The author also describes some curious crystals of leucite which 

 enclose foreign particles disposed in a concentric radiate form distinct from 

 the zonal arrangement which often obtains in this mineral. The microscopic 

 structure of elasolite from the zircon-syenite of Laurovig and Frederiksvarn, 

 in Norway, has likewise been studied by the author, who shows that the 

 mineral from these localities is never a pure substance, but consists of a 

 colourless mass in which are disseminated microscopic crystals of hornblende, 

 to which the green colour of the elaeolite may be referred. But perhaps the 

 most interesting of Zirkel's results is the discovery of microscopic tridymite — 

 Vom Rath's new species of silica — widely distributed through trachitic rocks. 

 In the rock of San Cristobat, near Pachuca, in Mexico, the mineral occurs in 

 colourless six-sided plates, having a length and breadth rarely exceeding 

 o - o2 m.m., the outlines being somewhat rounded, and the crystals grouped 

 together in a way which seems to be characteristic of this species. Tridymite 

 is also found in crystals, both microscopic and macroscopic, in the sanidine- 

 trachyte of the Drachenfels and Perlenhardt in the Siebengebirge on the 

 Rhine. 



*Leonhard und Bronn's Jahrbuch fur Mineralogie, U.S.W, Heft 7, 1870, p. 801. 



