ANNIVERSARY ADDRESS OF THE PRESIDENT. 73 



useful for varieties exceedingly poor in muscovite. At present we 

 are obliged to designate the hemicrystalline members quartz- 

 porphyry or quartz-felsite (I would call these elvanite, were not 

 the name so radically bad, or eurite, had not that term, in sym- 

 pathy with its etymology, been used vaguely) ; here also we wait 

 for a name*. Hyaline forms, as might be expected, are more 

 common than ever ; those in which individualized constituents 

 (commonly microlithic felspars) abound are called indifferently 

 quartz- trachytes, liparites, and rhyolites, though the last term is 

 generally applied to more compact and so less rough (or less 

 trachytic) varieties. The first term is open to the objection already 

 stated ; the second to this, that the rock abounds at many places 

 besides the Lipari Islands ; while the third perhaps indicates too 

 distinctly the appearance of having once flowed. If, however, 

 it were agreed to use trachytic as an adjective, and call trachytic 

 rhyolites the rocks usually named quartz-trachytes, the difficulty 

 would to some extent be avoided. At present the most glassy 

 members of this and the next group receive the names of pitch- 

 stone and obsidian, the former name being applied to rocks 

 with a resinous lustre, due probably to individualization of ex- 

 tremely minute microliths, and the latter to the most perfect 

 glasses, which also have a more distinct conchoidal fracture, that of 

 the pitchstones often being splintery. 



In the second group the name quartz-diorite is commonly applied 

 to the holocrystalline members, which may be defined as quartz + 

 plagioclase + hornblende (biotite being here decidedly less common 

 than in the other) ; but I think the name tonalite greatly preferable, 

 for as this rock appears to be far less common than granite, it may 

 be allowable to use the name of a locality where it occurs in a huge 

 and very typical mass. For marked varieties we should have the 

 names mica-tonalite and augite-tonalite, the latter probably being 

 very rare. Por the hemicrystalline division we have at present no 

 other name than quartz-porphyrite ; but for the hyaline division, 

 corresponding with the quartz-trachytes, we have fortunately the 

 name dacife. Of the most glassy varieties I have already spoken. 

 It may be remarked that free quartz appears to be less common 

 with the plagioclase felspars than with orthoclase, perhaps because 

 albite contains a higher percentage of silica than orthoclase. 

 Extremely glassy forms also may prove to be rarer in this case, 

 because, as I am informed, a soda-glass is more liable to set up 

 crystallization than a potash-glass. 



From the above remarks it will appear, not only that our nomen- 

 clature is at present in much confusion, due in part to the want of 

 any definite principles, but also that the inherent difficulties are 

 considerable, owing to the existence of transitional forms. At the 

 same time I believe that there is sufficient predominance of what I 



* Granophyre has been proposed by some distinguished German authors ; 

 but this term has been used in more than one sense. In fact, as will appear 

 from the latter part of this address, I believe the subgroup will have to be 

 rearranged, because its structure is often of secondary origin. 



