IGNEOUS ROCKS OF TASMANTA. 303 
4. A singular blending of principles of nomenclature 1; 
noticeable in the gabbro and peridotite families, where 
names have been given sometimes on mineralogical, some 
times on geographical, bases. Thus we have hornblende 
gabbro, mica gabbro, mica peridotite, and harzburgite, lher- 
zolite, &c. The terms are so familiar that to disturb them 
would be wanton pedantry. In giving names to new rock- 
varieties, I think geographical reasons are the most desn~- 
able. Physical peculiarities do not seem sufficiently dis- 
tinctive, as they may be repeated in different families, and 
the name loses its raison d’étre. 
With the above considerations im view, I submit a few 
remarks upon the rock-names in use in Tasmania. 
The country consisting so largely of mineral lands, which 
have been explored by mining operations, miners’ terms for 
all sorts of rocks are common, as might be expected. 
(a.) First comes the time-honoured designation of diorite 
for any obscure eruptive rock, though occasionally we have 
a wilder flight in the use of the term dioritic sandstone. In 
these days there is no excuse for applying the term to an 
unknown rock. It has a definite meaning in petrology, viz., 
a mixture of limesoda felspar with hornblende, mica, and 
occasionally pyroxene; and to use it in any other sense is 
misleading. Apart from the unexpected diorite in the Port 
Cygnet group, I do not know of any diorite proper in Tas- 
mania. The term epidiorite is only applicable to diabase 
altered by dynamometamorphism, and some obscure rocks 
in the North-Western district may eventually come within 
this definition. 
(6.) Secondly, porphyry enjoys a good repute among a 
mining population. Many English petrographers, and, I 
think, the United States Geological Survey officers, have 
discontinued the use of this term, and employ only its 
adjectival form, “ porphyritic.” In Germany porphyry is a 
porphyritic rock with orthoclase, and porphyrite, a rock 
porphyritic with triclmic felspar. So long as quartz-por- 
phyry is used as the name for a familiar rock-family, the 
word cannot be altogether excluded. American authors 
overcome the difficulty by employing the term aporhyolite 
for devitrified acid lavas. I fear it will be difficult to secure 
European acceptance of this. Fortunately, in Tasmania we 
are saved from an incongruous extension of the term, as 
we have no triclinic porphyritic rocks. The local designa- 
tion of the Port Cygnet trachytes, phonolites, syenites, &c., 
is “‘ porphyry,” and as a field-term is not objectionable, since 
