CORDIERITE BEARING-ROCK FROM BROKEN HILL, 219 
Apatite in small colourless needles and stouter prisms is present 
to a small extent, as also is iron pyrites in rounded grains, while 
ilmenite either partly or wholly altered to sphene and leucoxene 
is somewhat more abundant. All of the essential mineral com- 
ponents of this rock, with the exception of biotite, polarize in low 
colours, and for this reason thin sections of it are not particularly 
_ striking when viewed between crossed nicols. 
The felspars and quartz show strain phenomena in the form of 
wave extinction, and the whole appearance of the rock seems to 
indicate that it has suffered somewhat from the effect of natural 
stress. In the development of the cordierite, the rock upon the © 
southern side of the exposures seems to have played an important 
part, and field observation certainly leads one to the conclusion 
that it has arisen as the effect of contact-metamorphism, coupled, 
perhaps, with the general regional metamorphism which has so 
profoundly influenced the rocks over the whole of this district. 
The rock referred to as existing upon the southern side of the 
exposures, is now a rather felspathic amphibolite, more or less 
schistose in character, and it appears to be a typical case of the 
common alteration which certain basic eruptive rocks—e.g. dykes 
of gabbro or pyroxene diorite—suffer when subjected to more or 
less intense metamorphism. | 
The nomenclature of rocks is in itself a “rock” upon which 
many of the ablest of our geologists and petrographers have split, 
but few, I respectfully venture to consider, after due examination, 
would urge any strong objection to the rock, which I have had 
the honour of bringing under your notice, being termed cordierite- 
granulite. 
This photograph was taken from a thin section of the rock. The 
minerals represented are cordierite, quartz, felspar, biotite, and 
zircon. The cordierite shows in three grains—the large central 
one shows the false appearance of relief due to its border of more 
highly refracting decomposition product. It includes two zircon 
