THE PYROXENITE-AUGITITE GROUP 93 



(3) THE LIMBUEGITES 



This is a small group of lavas described by Rosenbusch in 

 1872 as occurring at Limburg, on the Rhine. The essential con- 

 stituents are augite and olivine with the usual iron ores. Struc- 

 turally they are never holoerystalline, but glassy and porphyritic. 

 The composition of the Prussian limburgite is given as below. 



Chemical Composition of Limburgite 



Constituents 



Silica (SiOg) .... 

 Alumina (AI2O3) . . . 

 Iron sesquioxide (FezOs) 

 Magnesia (MgO) . . . 

 Lime (CaO) .... 

 SodaCNanO) . . . . 

 Potash (K2O) .... 

 Water (H2O) .... 



Per cent 



42.24 



18.66 

 7.45 

 12.27 

 11.76 

 4.02 

 LOS 

 3.71 



101.19 



So far as known, the group has no representatives in the 

 United States. 



8. THE PYROXENITE-ATJGITITE GEOITP 



Here are included a small group of eruptive rocks differing 

 from the last mainly in the absence of olivine as an essential 

 constituent. They are represented, so far as now known, only 

 by the plutonic pyroxenites and effusive augitites. 



(1) THE PYROXENITES 



Pyroxenite, a term applied by Dr. Hunt to certain rocks con- 

 sisting essentially of minerals of the pyroxene group, and which 

 occurred both intrusive and as beds or nests intercalated with 

 stratified rocks. The author here follows the nomenclature and 

 classification adopted by Dr. G. H. Williams.^ 



Mineral Composition. — The essential constituents are one or 

 more minerals of the pyroxene group, either orthorhombic or 

 monoclinic. Accessory minerals are not abundant and are 

 limited mainly to the iron ores and minerals of the hornblende 

 or mica groups. 



^American Geologist, Vol. VI, July, 1890, pp. 35-49. 



