717 
‘‘ The former variety, owing to its coarse grain, would be called a nevadite, whi st 
the latter would be best described by the term sanidine-tracbyte ; both, however, are 
merely varieties of one and the same rock. The rock is intrusive througli the volcanic 
series, and sends out here and there dykes of no great thickness, and of a some\i a 
similar character to the rock forming the main mass. Hardly any apparent alteration las 
taken place iu the rocks through which the mass has burst.” 
(6) Mount Mandaeana. Sp. Gr. 2 '46. 
Colour white, with a tinge of blue. Microgranitic, holocrystalline. The whole 
rock is made up mainly of ground-mass, there being very few porphyritic felspars. 
Section— Shovift a little corroded mica, some minute specks of a mineral polar- 
izing in high colours, with a greenish, sometimes colourless, tint. A very ew 
porphyritic felspars, probably oligoelase. A very regular scatteiing of grains ot 
haimatite (or some translucent iron oxide) and a little interstitial glass spread over an 
area of ‘05 inch, and not occurring anywhere else iu the section. The mica shows comp e e 
absorption for rays vibrating parallel to lameUa when the section is normal thereto. 
(8) Pinnacle, lIiLLSBOEonon. Sp. Gr. 2’49. 
Colour, brownish-red. With marked fluxion-structure, porous and very fine-- 
grained. • i i i 
SeciioM.— Ground-mass highly felspathic, with well-maiked spheroidal stniotime, 
and but little quartz. A few specks of a mineral polarizing in high colours, and a tew 
water-clear sanidines. The angle of extinction right and left of the twinning line tor 
a twinned crystal is-right 9° 40', left 19° 30'; this crystal measures ’12 inch long by ’04 
inch broad, and is perfectly limpid. There are certain lines iu the section, on either 
side of which the microlites arrange themselves transversely. Whether this is a result 
of fluxion is not certain. I have not seen fluxion-structure exemplified in this way 
before. Mr. Maitland says; — , 
“ Another fragment of rock of this class is seen capping the Fiiilayson Mills, 
22 miles north-west of Mackay, and about miles from the sea-coast, where 
a sugarloaf -shaped peak, resting on granite, and rising to a height of about 100 eet 
above the summit of the hills.* The rock has a matrix of a purple-grey hue, with we - 
marked banded or ribboned structure, in which the lines of flow can often be seen o 
bend round the larger sanidine crystals embedded in the base. As a whole, the rocli 
is much more compact than the Mandarana trachyte, and, like it, forms rude y 
hexagonal columns.” 
(9) Alligatoe Ceeek, Saint Helens. 
Fine-grained brown rock. 
Section.— Qj reflected light shows ferrite, white dusty specks, and occasionally 
pyrites, without magnetite. Between crossed nicols there is no interstitial glass, an . i 
is difficult to say whether the much kaolinized felspar laths are embedded in a fels 
mass or not, since although, as a whole, the laths aro dmtmct, yet j 
power is used all definition disappears, the edges merging into a fine-grained gai u 
aggregate, sometimes polarizing brilliantly. The felspars are highly altered. 
them are filled with irregularly scattered grains of a very doubttu epidote ' 
sanidine is pellucid as usual. Calcite is occasionally seen in small ^ 
lamellie. The porphyritic crystals are represented by two or t hree w recks of p , 
• PI. 46, Eg. 2. (A.W.C.) 
