732 
perlite, puinice^ncl certain qiiartziferous traeliytic lavas, under tire terms rhyolite and 
liparite. The student should thereFore bear in mind the fact that the separation of 
the vitreous from the crystalline rocks refers merely to physical differences which the 
members of these two sub-classes respectively present, and does not imply any special 
difference in tlieir chemical composition. These physical differences depend upon the 
conditions under which solidification was effected, whether gradual or rapid. In the 
former case the molten mass would develop crystals, in the latter it would remain 
amorphous; it would, in fact, result in a more or less perfect glass. In these natural 
glasses it is, however, common to find crystallites and crystals, the former usually 
developed very completely, the latter less perfectly formed as a rule, since they generally 
present rounded houiularies, or their angles, if any exist, also appear rounded.” 
From Rntley’s remarks, as above quoted, and from the fact that the whole 
subject of the classification of rocks is so beset with pitfalls, it apjjeared safer to 
provisionally place the Mackay quartz-trachytes under the rhyolite group.* Subse- 
quently they Tiiay be re-classed, when the rock collections are more representative. 
I have no wish to evade the classificatory difBculty, but after quoting Professor Hutton’s 
remarks on the difficulty of classifying rocks, I shall leave the reader to judge of the 
wisdom of waiting for further developments. He says f : — “ The confusion, indeed, has 
become so great that some petrologists discard names as much as possible, and in their 
place give a list of the minerals composing the rock. This plan has the merit of not 
adding to our confusion, but it will not help us out of it. And when wo read of 
‘ plagioclase-augite-olivine-mica rock,’ or of ‘ felspar-pyroxene-magnetite-garnet rock,’ 
our patience is well-nigh exhausted. 
“ Natural science progresses by the comparison of objects, and when objects are 
numerous they must be named, if for no other purpose, still for indexing ; so that an 
observer may readily find descriptions of similar objects for comparison, and ascertain 
what others have said about an object similar to the one he may be studying. As books 
get more and more numerous a uniform nomenclature becomes of more and more 
importance from the single point of view of indexing, and until some uniformity in 
rock nomenclature has been attained we cannot expect any very great advance in 
petrology.” 
The rhyolites are poorly represented in this Collection, but are generally typical. 
Particular attention is drawn to the Nell Isle sample, No. 108, which is so beautifully 
characteristic that the artist has drawn it in three different lights. For those readers 
to whom the subject is new they afford excellent illustrations of fluxion-structure and 
rock-texture. 
The Mackay sample, No. 58, shown on Plate No. 62, fig. 2, is also a good type of 
the group. 
The Mackay quartz-traehites are fairly interesting, but as type rocks they are 
not above the average. What makes them of special interest is that they illustrate 
Departmental work, having been prepared in London by a professional, and are about 
as thin as rock-sections can possibly be, which is the desideratum in petrography. 
The Cloncurry obsidian calls for special notice, owing to its anisotropic pro- 
perties. It faintly polarizes between crossed nicols, and it is remarkable that the 
section is free from microlites, crystal growths, and gas-pores. Zirkel says such vol- 
canic glasses are extremely rare. Finally, the single example of taehylyte merits a 
* Mr. Maitland infomis me that the Mackay trachites- exhibit, in the field, almost all degrees of 
texture, from coarse trachites at one end of the series to nearly trachite glass at the other. 
t “The Eruptive Rocks of New Zealand.” A Paper read before the Royal Society of New South 
Wales, 7th August, 1889, and published in their journal, vol. xxiii., p. 102. 
