1870.] 



MACFARLANE — ON CRYSTALLINE ROCKS. 



309 



Augitic porphyry from Fassatha.l 

 in Tyrol 



Xophelinite from Wiekensteiu in 

 Lower Silesia, — Lowe 



OXYGEN RATIO. 

 Bases. 



1 



Silica. 

 1-391 



1-347 



Quantity ot 

 Siliia in HhJ 

 parts rock. 



4,') -or. 



41-87 



The uumber of analyses of these basic rocks being somewhat 

 limited, it is not possible to arrive at their average silica contents 

 so closely as in the case of the neutral and silicic rocks. These 

 instances, however, she\Y that the oxygen ratio l*3o3:l corresponds 

 to a percentage of about 43. 4G silica. Rocks thus constituted 

 being two-third silicates, might be conveniently called sub-silicates, 

 and, in contradistinction to the .silicic series, might be termed 

 the basic rocks. 



Between the basic and neutral rocks, on the one hand, and the 

 latter and the silicic rocks on the other, there exist many other 

 rocks of intermediate composition and forming gradual transitions 

 between each of the series, which have been more minutely 

 referred to in the foregoing. It thus becomes possible to point 

 out a series of rocks passing gradually from the basic extreme to 

 that of acidity in composition, not only for each of the granular 

 porphyritic and trachytic order of rocks, but also for every variety 

 of texture specified in the preceding chapter. The following 

 Table gives an arrangement of these various series of rocks and 

 an exhibition of the distinctive character.s as to texture and 

 chemical composition possessed by each. In constructing this 

 table, it has been found that by limiting the variations in silica 

 contents of each family to 7 per cent, very correct lines of 

 separation may be drawn betwixt them : — 



Table I, 



hihowing the General Chemical Composition of the Families 

 of Original Rocks. 



Orclcr of Texture. 



I Baste Kocks, 



'. (subsilicates), 

 containing lest 

 ! tliau 49 per 

 j cent. Silica. 



I. Coarse and siuall-! 



prai' ed Anorthosite- 



II. Schistose iBasic schst. 



III. Slaty. 



IV. Porphyrite Augitic por- 



V. Variollte '■ ../.'..?..' 



VI. Flne-.'iained i «asal\ 



VII. Trachy.lc Nephellnlte. 



Vm Volcanic INephellnite 



. lava. 



Basous Rocks 



containing 



from 49 to 55 



per cent. Silica 



Orepnstonf. 

 Hornblende 



cbist;. 

 Greenstone 



slate. 

 Green-tone 



porphyry. 

 Variollte. 

 Trap. 

 DolerltP. 

 Dolejlc* lava 



Neutral Rocks,' Siliceous 



(monosilicatesh Rocks, Silicic Rocks, 



containing | containing containing 



from ; 6 to 63 ; from fiS to 70 , more than 70 



percent. Silica, per cent. Silica [per cent. Silica 



Syenite. 

 Syenite 

 schist. 

 Clay slate. 



Melaphyre. 



Grantlte. 

 Gneiss. 



Siliceous 



slate. 

 Porphyrite. 



iVar.basaltite 



IBasaltite. Eurite. 

 ■Andesite. ,Tracbyt*?. 

 ;Acde8lte irachytlc 



lavd. lava. 



Granite, 

 !Guil..site. 



S'llclc slate. 



Felsitic por- 

 I pliyry. 

 iSpuarolite, 

 jFelslte. 

 RhvoMte, 

 lObsldlau, 



