302 PROCEEDINGS OF SECTION C. 
been made is unwarranted. For instance, to speak of the 
Mesozoic diabase or dolerite in Tasmania as greenstone or 
trap in a scientific assembly is unpardonable. The field- 
terms may be excusable, however, in accommodating our- 
selves to our audience. 
3. As a rule a single substantive is preferable to a descrip- 
tive term, because it is less cumbrous. The more cumbrous 
a term is, the less suitable it is as a working imstrument. 
Thus “granite” is better than “ orthoclase-quartz-mica 
rock.” But*in subdivisions a descriptive nomenclature is, 
I submit, desirable when not allowed to grow unwieldy. For 
example, I prefer biotite-granite to granitite, because it is 
self-explanatory, as well as in harmony with the other sub- 
divisions, hornblende-granite, muscovite-granite, &c. 
Elvan is a single term, and may be used advantageously, 
if we know what we mean by it; but my personal preference 
is for the more comprehensive, and also scientifically more 
precise, “granite-porphyry.” The “blue elvans”’ of 
Cornishmen are chiefly dykes of diabase, 
Quartz-porphyry is a descriptive term which I would con- 
fine to devitrified rhyolites and other effusive forms of the 
granitic magma. If its ground-mass is felsitic, it may be 
called felsophyre; if granophyric, granophyre; if micro- 
granitic, microgranite. If it carries no phenocrysts, it is 
felsite. Granite porphyry dykes sometimes have an identi- 
cal structure, which is assumed, too, by the marginal parts 
of granite masses. In such cases it is permissible to speak 
of the quartz-porphyry facies of granite, &c. Thus, if the 
quartz-porphyries and felsites of our West Coast range are 
devitrified rhyolites, they may be regarded as quartz-por 
phyries purely and simply; while, if they are intrusive 
tongues from granite, we have to do with a quartz-porphyry 
facies. 
In pursuance of the descriptive method, I would banish 
the terms tachylyte and hyalomelane, and substitute hyalo 
basalt. The same prefix is available for the glassy forms of 
rhyolite, trachyte, and andesite, doing away with the eub- 
stantival terms applied to the former (pitchstone, vitrophyre, 
obsidian, &c.). Hyalo-rhyolite, hyalotrachyte, and hyalo 
andesite are far more precise; or, if wished, rhyolite-glass, 
trachyte-glass, andesite-glass may be used, as suggested by 
Judd long ago. 
When these glassy rocks bear plentiful phenocrysts, the 
adjective “ porphyritic’’ may be added. ‘For instance, in- 
stead of the old term “ pitchstone-porphyry,” we can use 
“porphyritic hyalo-rhyolite” or ‘‘ porphyritic rhyolite-glass.” 
