70 PROCEEDOGS OP THE GEOLOGICAL SOCIETY. 



however, many gabbros haye undergone much alteration, the felspar 

 being changed into a sanssuritic mineral, the diallage into some 

 form of hornblende, it would be well to restrict the term euphotide 

 to these. Again some authors nge the terms dolerite, anamesite, and 

 basalt for rocks which, chemically identical, and all holocrystalline, 

 differ in the coarseness or fineness of their grains, so that the last 

 term is applied to a rock which either may be holocrystalline or 

 may retain a glassy base. It would be convenient, then, to restrict 

 the term dolerite to the holocrystalline variety, using the epithet 

 coarse-grained or fine-grained as the case ma} be : to apply the name 

 anamesite to the hemicrystalline varieties (very few and local, I 

 suspect) ; and to include in the term basalt all that retain a glassv 

 base, i. e. the magma-basalts and glass-basalts of some authors. 

 It may be found convenient to use the name magma-basalt (as does 

 Bofick<) for those in the base of which the felspar remains unindi- 

 vidualized. The name tacliyhjte has been applied (as is well known) 

 to varieties where almost the whole material remains in the form of 

 glass, and the term may be conveniently retained, if we remember 

 that it is only a marked variety of the glass-basalt division. 



Two rather limited groups of uncertain position come next in 

 order, the phonolites and the teplirites. The former group is essen- 

 tially characterized by the presence of nepheline and of felspar, 

 which is commonly, in part at least, orthoclase, the latter by leucite 

 and felspar (more commonly plagioclase) ; but there are several 

 accessories and many varieties, as may be seen from an examination 

 of Bofickv's divisions of the former group alone. It also is one 

 whose nomenclature is in great confusion. Among the older mem- 

 bers we have zirkon -syenite, eleeolite-syenite, nepheline-syenite, 

 foyaite, ditroite, miascite, while the one term phonolite covers 

 all varieties, whether holocrystalline or not, among the more modern. 



The group being, to a large extent, a transitional one, differing 

 from that last in order by a higher percentage of silica and of 

 alkalies, and a lower percentage of lime and magnesia, we can 

 hardly hope to secure very marked type-forms ; but ]3erhaps the name 

 foyaite might suffice as a specific term for the holocrystalline 

 varieties ; while in the case where the characteristic mineral is very 

 distinctly the variety elseolite rather than nepheline (which does not 

 appear to be the case with all foyaites), we might prefix the term 

 elseolite. We should thus have as varieties elaeolite-foyaite, nosean- 

 foyaite, &c. It will, however, be a subject for consideration 

 whether it may not be desirable, after the analogy of the groups 

 which follow, to divide those in which a plagioclase felspar pre- 

 dominates, from those in which the felspar is chiefly orthoclase ; 

 but as my opportunities of studying this group have been rather 

 limited, I will not venture an opinion on this point. Of the 

 tephrite group 1 must speak yet more guardedly; for although I 

 possess specimens, I have not had the opportunity of paying much 

 attention to the rock. The name, however, is convenient to denote 

 plagioclase-leucite rocks, and should be applied to rocks in which the 

 former mineral predominates, and the percentage of silica is higher 



