ANNIYERSAKT ADDRESS OF THE PRESIDENT. 7 I 



than in the leucitite group and the forms intermediate between it 

 and the doierite group. An examination of a table of the chemical 

 composition of minerals shows that with a high percentage of alumina 

 compared with the silica, we shall obtain anorthite in rocks rich in 

 lime and poor in alkalies ; labradorite with a lower percentage of 

 lime, and a moderate proportion of alkalies (chio% soda) ; nepheline 

 with a higher percentage of alkalies, soda still predominating ; and 

 leucite with an exceptionally high percentage of potash. 



The next two groups contain a higher proportion of silica, 

 rarely less and often more than 60 per cent. Normally they may 

 be defined as felspar with hornblende, though in certain divisions 

 augite more commonly replaces the latter; biotite is not unfrequent, 

 sometimes dominant over the pyroxenic mineral; at times hyper- 

 sthene is an important accessory, taking the place of the unisilicate 

 olivine among the (more basic) dolerites. Although transitional forms 

 may be found between these groups, it is convenient to designate 

 the more typical representatives by different names ; thus the ortho- 

 clase-hornblende group may be called, after the holocrj^stalline 

 form, the syenites, the plagioclase-hornblende group, the diorites. 

 Hemicrystalline and glassy forms of both are common, but it is by 

 no means easy, especiall)' in the case of the latter, to separate them 

 without chemical analysis, though the character of any individua- 

 lized felspar crystals is often a great help. I may remark, however, 

 that not a few of the so-called hemicrystalline forms are in reality 

 holocrystalline, though very fine-grained, and others can almost be 

 proved to have assumed a hemicrystalline structure by secondary 

 processes, so that in reality they should be placed in the meta- 

 morphic division ; but leaving that distinction to await the results 

 of further study, we may recognize the following divisions of 

 the syenite group : — holocrystalline ; orthoclase -f- hornblende = 

 syenite; if mica replaces the hornblende wholly or to a marked 

 extent, mica-syenite', if augite — augite-syenite. To the last rock, by 

 no means a typical member of the group, as it contains much plagio- 

 clase felspar, the name nionzonite has been given. The hemi- 

 crystalline division contains a considerable number of the rocks 

 macroscopically grouped as felstones. By many authors they are 

 called orthoclase-porphyries ; but the name porphyry, as I have 

 stated, being objectionable, and a binary term for a group being 

 awkward, I should prefer to call them felsites. Then those in 

 which orthoclase or any other mineral was conspicuously present 

 might be called orthoclase-felsites, hornblende-felsites, mica-felsites, 

 or the like. Hyaline forms are now becoming abundant, though 

 commonly the glassy base contains a large number of individualized 

 minerals. These are generally named sanidine-tracliytes ; but as 

 the term trachyte is used like felstone for grouping in the field, its 

 use in this more limited and strictly definite sense is objectionable, 

 and, as before, the compound term is awkward. It is no part of my 

 present purpose to attempt to coin new names, so I content myself 

 with expressing a hope in this and like cases that a geological 

 congress will some day invent and authorize one ; till then I must 



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