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masses appear in juxtaposition with the hornhlende. They are also amorphous, and 
may he graphite. Occasionally brown hornblende occurs, which, as usual, is very 
strongly dichroic. Epidote occurs amongst the altered hornblende. It is white, and 
polarizes of course brilliantly. Some few yellow varieties occur ; and in one of the 
white epidote masses, a few microliths, that favour the monoclinic felspar variety, occur, 
shaped thus : — 
In the reddish varieties of the porphyry, ferrite is plentiful. 
Mr. Jack, in a letter to me, dated Croydon, 4th December, 1889, states that this 
rock “ forms the hills lying side by side with the syenite forming the flats, and in which 
the reefs of Croydon proper lie. The junction I’uns N.AV. to S.E., just a few chains 
oS the Queen line of reef for two or three miles, and so straight as to suggest a 
fault.” 
“I have never seen the syenite intruding thi-ough the quartz porphyry, or vice 
versa. Then again the quartz porphyry shows fluxion-structure in a manner 
suggesting a true igneous rock. Further, the quartz porphyry at times weathers so as 
to show apparently purely siliceous, rounded enclosures, suggestive of an altered 
conglomerate. The quartz porphyry (as well as the syenite) is full of little nests of 
graphite, also suggestive, I think, of metamorphism.” 
Nos. 79 ATXD 80. G-rf.at Noetherii' Tin Mine, Herberton (Queensland). 
No. 79 is from three hundred feet below the surlace. It is a grey elvan, with 
glassy spots of quartz, and occurs as a dyke in granite. 
Section . — Elvan with felsitic base. This is really a quartz-porphyry : a 
mierocrystalline granite base, with porphyritic quartz. The felsitic matter, under the 
highest powers, carries light-green specks, quartz, and felspar, but there is no absolute 
certainty about these minerals except that the green specks show faint absorption 
when the stage is rotated over the polarizing nicol without the analyser. The ground- 
mass is dusty, and carries rare pyrites. The quartzes show corrosion and rounded 
contours, with the usual bays, inlets, and islands filled up with the ground-mass. 
These quartzes are from 0'2 inch to 0'4 inch, and one is 0’8 inch in diameter. The 
specific gravity of the rock is 2'603, 
No. 80 is from the same dyke cropping out at the surface, and is precisely the 
same as No. 79, but for the reddish colour due to weathering. The specific gravity of 
the rock is 2'559. 
Section . — Felsitic matter much more opaque and dusty, and quite impossible to 
resolve. Porphyritic quartz, whose edges and angles are much less rounded and corroded 
than is the case with No. 79. They measure from 0 3 inch to 0'5 inch. The felspars 
are represented by one much altered twinned plagioclase, whose angle of extinction, 
between the two sets of lamellm, is only 1° or 2°. It is TOO inch long, is often twinned, 
