REVIEWS 739 



defined monzonite as a peculiar rock varying between syenite and 

 diorite as extremes. His name was for 2i geological unit, however, and 

 was not a petrographic definition. Dolter in 1875 adopted a similar 

 usage ; hand specimens of monzonite were syenite, diorite, gabbro, 

 augitfels, or diabase. Rosenbusch, Teall, and Zirkel refer to mon- 

 zonite as augite syenite. Nearly all writers have agreed in regarding 

 the rocks of this localit}- as a series of intimately related members, 

 ranging from acid to basic chemical composition, but have differed 

 as to which is the main type. 



In the laboratory the author's collection of specimens was found 

 to fall into two groups — an acid group with 50-60 per cent, of silica, and 

 a basic group with less than 50 per cent, of silica. The former group 

 is the main one and embraces the monzonites proper, which are ortho- 

 clase-plagioclase rocks; while the latter are pyroxenites, and are 

 merely peripheral facies of the monzonite, into which they grade. 

 The term monzonite is thus given definite petrographic significance as 

 the name of a transition or intermediate group of rocks between the 

 orthoclase rocks (syenites) on the one hand, and the plagioclase rocks 

 (diorites) on the other. The former method of uniting such a transi- 

 tion group with one or other of the limiting members of the series 

 would, in this case, fail to express the most characteristic thing about 

 these rocks, viz., that they contain plagioclase and orthoclase in about 

 equal proportions. 



Chemical investigation still further shows the need for a special 

 name for the group. The probable range in silica percentage in all 

 the monzonites is from 62-60 per cent, down to 50-48 per cent. This 

 excludes all granites, quartz diorites, and quartz syenites from the com- 

 parison. The gabbros, too, are mostly more basic, have lower alkali 

 contents, higher lime, and generally higher magnesia and iron oxides. 

 A direct comparison, then, is necessary only with the syenites and 

 diorites. The nepheline syenites have about the same silica contents 

 as the monzonites, but are otherwise very different, as is shown by the 

 following ratios deduced from thirteen analyses of nepheline syenites, 

 and five of monzonites : 



Nepheline syenite - - CalNajO: KoO: : i :4 :2 



Monzonite . . - CaO: NaoO : KoO: : 10: 4 : 3 



The chemical affinities of monzonite with the syenites and diorites 

 are shown in the following table. All the analyses used for compari- 



