729 
felspars are large, measuring from '05 to ’1 inch long. The angle between the axes of 
extinction for the two sets of twinned hiinellse in the largest felspar shown in Plate 60 
is 14°. 
No. 177. MorsT BiscnoiT (Tasmania). 
E.. L. Jack’s Collection. Sp. Gr. 2'7B3. 
Colour, a dirty-white rock, with a few dark specks. 
Section. — Ground-mass inicrogranulitic. Quartz and felspars with prophyritic 
quartzes, not very much corroded. Pelspar forms filled up with felsitic matter and a 
radially-arranged mineral, highly coloured between crossed nicols. The quartzes carry 
inclusions with moving bubble.?, often dihexahedral. Dirty greeuish-blaek, semi- 
translucent grains ai-e pretty evenly distributed over the section. They polarize feebly 
under the i-inch objective. 
Note. — The folli)vving is an extract from the “ Physiography of the Rock-making Minerals ” (p. 292), 
explanatory of Eosenbusoh’a I’ormulaj for Plagioclase Felspars, I'eferred to in the foregoing notes (pp. 703 
and 721). 
“The chemical composition of the theoretical albite is NajO, AljOj, 6 SiO»=Na», Al», Si^, 
Ojo=Ab ; that of anorthite, 2 CaO, 2 AljOj, 4 Si0.i=Ca2Al2, AU, Si*, 0]e=An. All other lime- 
soda felspars, then, are iaomorphou-s mixtures of albite and anorthite= Abn , Anm. Of the many possible 
mixtures, certain ones occur more frequently, and have received p.articular names. If these be enlarged by 
the addition of those compounds closely connected with them, then, following Tschermak, the lime-soda 
felspars or plagioola.ses m.ay be brought into the following table ; — 
Albite series embraces the compounds Ab,, Anu — Ab,, An,. 
Oligoclase series 
Andesine series 
Labradorite aeries 
Bytownile series 
Anorthite series 
Abo, An, — Abj, An,. 
Abo, Anj — Ab*, Anj. 
Ab,, An, — Ab,, An,. 
Ab,, An^ — Ab„ Anj. 
Ab,, An,— Ab„, An,.” 
SUMMARY. 
The foregoiug notes throw no new light on the science of petrography. Indeed, 
one is struck with the microscopical similarity of the crystalline rocks of Australia, 
America, and Europe. 
In many of the granites almost the same language can be used in describing the 
structure and constituting minerals of the innumerable varieties of the group as is 
employed by authorities like Eosenbusch, Eutley, Teall, Zirkel, and others. 
This is no deterrent, however, to the student, because the very sameness of our 
crystalline rocks with those of other parts of the world proves the uniformity of those 
chemical and physical forces which operated so powerfully in their formation and 
subsequent mutations. 
In all branches of science the recording of facts is the preliminary work. The 
arrangement and classification of these facts generally falls to the lot of a distinctly 
different type of wmrker. I have endeavoured to contribute something to the pre- 
liminary work. 
The rocks described are divided into six groups, viz.: — Granites, Ehyolites, 
Basalts, Dolerites, Diorites, and Porphyrites. 
GRANITES. 
The quartzes of this group do not radically differ from those of the ordinary typical 
granites. Allusion may be made to the long, brilliant, colourless needles, particularly 
noticeable in the Croydon granite. They are not plentiful, possibly they are rutile, but 
might be apatite, though from their solitary habit I think the first more likely, as 
suggested by Teall in describing the Scotch granites. Many quartzes in this group 
carry such inclosures. • 
The hornblendes, both green and browm, are sometimes fringed with epidote. 
Occasionally a bluish-green envelope surrounds a deeper green or brown variety. It is 
