ANNIVEKSAKY ADDRESS OF THE PRESIDENT. 69 



jpicrite preferable to that of iKditiopicrite^ which has been given 

 by many authors. To this group probably the eulysites of some 

 authors (garnet-bearing peridotites) should be referred, and lim- 

 huryite would represent the hyaline form. I am not aware that a 

 hemicrystalline form has been recognized, certainly it has not been 

 named. 



As subgroups or intermediaries between this last group and the 

 next great one, we may regard the norites or liyperites, composed 

 ehieliy of a plagioclase felspar (anorthite or labradorite), with an 

 eustatitic mineral and perhaps a pyroxenic ; the corsites, anorthite 

 hornblende rocks (perhaps we might use eucrite for the anorthite 

 augite) ; and the trohtolites, containing the same felspar with olivine 

 and but little of a pyroxenic mineral. It is perhaps well to retain these 

 names, but it must be remembered that they have hardly a generic 

 value. It may here be worth while to call attention to the fact 

 that many of the so-called hypersthene rocks, e. g, the gabbros of 

 Skye, and of Carrock Fell (Cumberland), either do not contain hyper- 

 sthene, or have it only as a very rare accessory, while in others 

 it is not more conspicuous than a pyroxenic constituent, so that 

 there is no vahd reason for calling them more than hypersthene 

 (or enstatite) dolerites or gabbros. 



We come then next in order to a most important group, that 

 which contains, as constituents, a plagioclase felspar, commonly 

 labradorite, a pyroxenic constituent, usually augite (or occasionally 

 diallage), and often olivine *. To this group many names have 

 been given, and various subdivisions of it have been proposed. Some 

 authors extend it so far as to include rocks in which either nepheline 

 or leucite replace the felspar, calling them respectively nepheline- 

 dolerites, &c., or leucite-dolerites, &c. ; some distinguish those in 

 which olivine is present from those from which it is absent. Por 

 myself, while fully admitting the relationship of each of the former 

 pair to the normal dolerites, and the frequent existence of interme- 

 diate forms, I think that the marked chemical differences (in the 

 large percentage of alkalies) justify us in separating them from the 

 felspar-dolerite and in retaining the old names, nej^lielinite and 

 leucitite. Some, again, using geological age as a distinction, apply 

 the name diabase to a rock if it is Pre-Tertiary, and dolerite^ if of 

 later age ; this, of course I could not accept, but should propose to 

 give the former name to dolerite in which marked alteration, sub- 

 sequent to the original consolidation, has taken place. Others, 

 again, apply the name gabhro to coarse-grained varieties in which 

 the pyroxenic mineral is wholly or chiefly diallage. There is much 

 convenience in the use of this term for a variety often so well marked 

 in the field ; but there is at present doubt as to the classificatory 

 value of diallage, some authors of great weight regarding it as 

 really an altered condition of augite. Provisionally, however, we 

 may retain it, remembering that it may be logically indefensible. As, 



* I shall, for breyity, not again refer to the presence of a member of 

 Division I., because some oxide of iron, magnetite, haematite, or ilmenite is to be 

 found in every rock, though usually more abundantly in the more basic. 



VOL. XLI. Jl 



