GEOLOGY. 231 



One of the most important additions to litliological literature dur- 

 ing the past two years is the vohime on Mineralogie 3IicrograpMque 

 Bodies Eruptives Francaises, hy Fouque and Levy. The vohinie, though 

 large, is, however, simply an introduction, and treats of microscopic 

 minerals and methods of investigation. The volume is accompanied by 

 a large atlas of fifty-five plates ; this contains lithographic and photo- 

 graphic reproductions of sections as seen under the microscope — pic- 

 tures which in beauty and accuracy have never heretofore been equalled. 



A question extensively handled in this volume is the means for the 

 determination of optical properties iu chance ."sections, while the work of 

 Rosenbusch aimed more to show how one might find sections cut in defi- 

 nite directions, and then determine their optical properties. By a very 

 elaborate study, rouqu<5 and Levy deduce what properties sections should 

 have which might be cut in a multitude of ways through a crystal of 

 a given kind. This subject for the feldspars had been earlier opened 

 by Professor Pumj^elly, in our country. 



Yet however accurate these calculations may be, and how far beyond 

 criticism are the results, there are many practical difficulties which will 

 cause this method to be applied only to a most limited extent. Much 

 more practical and fruitful iu results is the method which, though not 

 original with him, Thoulet has applied to the mechanical separation of 

 rock ingredients. A solution of the iodide of potassium will dissolve 

 mercury iodide, and the resultant fluid is heavier than the more common 

 rock ingredients, which consequently will float upon it. By addition of 

 water the density of this fluid can be reduced to any desired point, and 

 the various constituent minerals that compose the powder of a rock 

 which is thrown into and mixed with this fluid can be made to settle 

 out one by one iu a state of purity and fit for analysis. This method 

 has already yielded practical results ; one of these is the demonstration 

 that rocks are even more comiflex in mineral composition than had 

 been supposed. 



This brings us back once more to the question of the feldspathic con- 

 stituent of rocks, which is of much importance, because feldspar is the 

 most generally distributed of all rock constituents. Many methods 

 have been proposed for their determiuation. Optical methods usually 

 allow potash feldspar to be distinguished from the other species, but 

 if more than one of the other kinds be i)resent, optical methods ouly 

 allow the one with greatest obliqity of elasticity planes to be detc^rmiued. 

 Consequently, rocks have been usually determined as being composed 

 of one species of triclinic feldspar with other minerals, or of one species 

 of triclinic feldspar with orthoclase and other minerals. Fouque, in his 

 work on Santorin, indicated that the feldspathic constituent of rocks 

 was more complex, and the application of the specific gravity fluid bids 

 feir to i^rove that this comjilexity is quite general. 



