18 A. W. HOWI'TL ON THE PHYSICAL GEOGRAPHY AND 
ry 
ig. 5.— Contact of Granite and Silurian, Orr’s Creek, Dargo Flat. 
(a) Ternary felspathic granite, very much decomposed. 
(b) Altered Silurian. Under the lens appears granular felspathic, with minute 
black specks (mica ?). 
(c) Fine-grained granite vein, about 3 feet wide, consisting of felspar, quartz, 
black mica. This is sharply defined as regards the bounding rock, which 
is similar to (f). 
(dz) Altered Silurian. In places micaceous, like (f); elsewhere indurated, 
finely crystalline, bluish black in colour. Traces of planes of deposit recog- 
nizable by lighter or darker undulating lines. 
(e) Decomposed felspathic vein or dyke, like (g), but more granitic. About 
12 feet wide. 
(f) Metamorphic rock, scbistose, and the joints lined with silvery mica. This 
seems to embrace and overlie (¢). 
(gq) Felstone vein or dyke, with minute patches of a foliated black mineral. 
(z) Indurated slates and sandstones (Silurian), as seen down the course of Orr’s 
Creek (auriferous). 
{k) Granitic vein crossing (d), a few inches in width; at the sides rather coarsely 
erystalline-granular, of yellowish orthoclase and translucent quartz. In 
the centre the same, but coarser, and with aggregations of silyery mica and 
black schorl. 
Note.—In the above sketch the lateral extension of the rocks has been con- 
densed in order to bring the whole section into view; but the features have 
‘been preserved as faithfully as possible. The apparent dip is due to the section 
‘ying-at an angle with beds which are in reality vertical, or nearly so. 
As the granites of the series (6) have invaded, cut off, absorbed, 
or altered the Silurian strata, so also have extensive extrayasations 
of quartz-porphyries and of rocks connecting them with the true 
granites taken place among the sedimentary strata. The effects 
produced are, however, far less marked than in the granites. 
The quartz-porphyries all agree in having a felsitic or somewhat 
erystalline-granular base, in which are usually porphyritic crystals 
or patches of orthoclase, and in all cases crystals or crystalline 
grains of quartz. The colour varies from almost white in restricted 
localities to shades of yellow, red, and purple. Some varieties are 
earthy, while others are highly silicified. The extreme forms are, 
on the one hand, a rock which might be classed as a somewhat 
erystalline form of binary granite (Mitchell River); and, on the 
other hand. a highly porphyritic reck, in which the orthoclase crystals 
are over an inch in length and often distinctly formed, and the 
quartz-crystals large and often in very regular double pyramids 
without any apparent intervening prism (Mount Taylor). The total 
area occupied by quartz-porphyries falls, no doubt, somewhat short 
