I 



PETROLOGY. 205 



Elementary instructions for determining minerals in thin rock- 

 sections, the diflferent appearance under polarized light, according to 

 the crystalline system, giving the most important aid. E. B. T. 



Frazer, Prof. P. A Study of the Igneous Eocks. Iron, n. s. vol. viii. 



p. 747. {Amer. Inst. Min. Eng.) 

 Describes the methods of study. 



Garrigon, Dr. Etude d'un Ciment Metamorphose par la Source 

 Bayen de Luchon. [Cement altered by the Mineral Waters of 

 Bagneres de Luchon (Pyrenees).] Compt, Bend. Assoc. Fran^. 

 1875, pp. 677-079. 



Analyses of the altered and unaltered cement. 



Gasparin, P. de. Sur la formation du sol arable dans les terrains 

 granitiques. [Formation of Arable Soil on Granites.] Bull. ISoc. 

 Agric. France, t. xxxvi. p. 523. 



Geinitz, Dr. Eugen. Ueber einige Griinschiefer des sachsischcn 

 Erzgebirgcs. [Green Slates of the Saxon Erzgebirge.] Min. Mitth. 

 Heft iv. pp. 189-206. 



Detailed description of the so-called green slates, which occur, asso- 

 ciated with phyllites, in the Muldenthal between Aue and Stein, and 

 also near Tharandt and Herzogswalda. The " hornblende green-slates " 

 consist of hornblende, epidote, titanitc, quartz, orthoclase, plagioclase, 

 and sometimes chlorite. In the " chlorite green-slates " the horn- 

 blende is replaced by primary chlorite. Sahlite is present in some of 

 the hornblende varieties. E. W. B,. 



Gilmore, Col. Report on the compressive strength and specific 

 gravity of the building stones in the United States in most general 

 use. In the Annual lieport of the Chief of Engineers to the Secre- 

 tary of War, for the year 1875. 



Gooch, Frank A. Ueber rulkanische Gesteine der Galapagos-Inseln. 

 [Volcanic Bocks of the Galapagos Islands.] Min. Mitth. Heft ii. 

 pp. 133-140. 



Describes the microscopic characters of some lavas from the Gala- 

 pagos Archipelago. A limestone-breccia, containing fragments of 

 altered lava, shows the formation of glauconite or a similar substance ; 

 and it is argued that this mineral and similar silicates may be regarded 

 as products of the decomposition of volcanic minerals, even when oc- 

 curring in sedimentary rocks like the glauconite-bearing Cretaceous 

 beds. E. W. It. 



. Bemerkungen iibor die Pcchsteine von Arran. [Pitchstones of 



Arran.] Min. Mitth. Heft iii. pp. 185, 186. 



Sections of pitchstone from Lamlash and Clachland Point contain 

 belonites which exhibit dichroism, and which are therefore referred to 

 hornblende. E. W li. 



