96 



EOCKS FORMED THEOUGH IGNEOUS AGENCIES 



has a glassy base, in which are embedded the crystals of augite 

 and iron ores- The composition of an augitite from the Cape 

 Verde Islands, as given by Roth, is as below : 



Chemical Compositioit op Augitite 



Constituents 



Silica (SiOa) .... 

 Alumina (AlgOs) . . . 

 Iron sesquioxide (FeaOs) 

 Magnesia (MgO) . . . 



Lime (CaO) 



Soda (Na20) .... 

 Potash (K2O) . . . . 

 Water (H2O) .... 



Pee cent 



41.83 



18.60 



16.11 



4.98 



11.83 



4.70 



2.47 



0.01 



101.43 



9. THE IiEUCITE-NHPHELINE BOOKS 



Under this head are grouped two small but interesting groups 

 of effusive rocks, having, so far as known, no exact equivalent 

 among the plutonics, and characterized by the presence of leu- 

 cite or nepheline, which here seem to play the role of feldspars 

 as essential constituents. In detail they are as below : 



(1) THE LEUCITE EOCKS 



Mineral Composition. — The essential constituent is leucite 

 and augite. A variety of accessories occur, including biotite, 

 hornblende, iron ores, apatite, olivine, plagioclase, nepheline, 

 melilite, and more rarely garnets, hauyne, sphene, chromite, and 

 perowskite. Feldspar as an essential fails entirely. 



Chemical Composition. — The average chemical composition as 

 given by Blaas^ is as follows: Silica, 48.9%; alumina, 19.5%; 

 iron oxides, 9.2%; lime, 8.9%; magnesia, 1.9%; potash, 6.5%; 

 soda, 4.4%. 



Structure, — The rocks of this group are, as a rule, fine 

 grained and only slightly vesicular, presenting to the unaided 

 eye little to distinguish them from the finer-grained varieties 

 of ordinary basalt. 



Colors. — The prevailing colors are some shades of gray, 

 though sometimes yellowish or brownish. 



^KateeMsmiis der Petrograpbie, p. 117. 



