Page:Quarterly Journal of the Geological Society of London, vol. 35.djvu/44

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A. W. HOWI'XT OH THE PHYSICAL GEOGRAPHY AND

Fig. 5. — Contact of Granite and Silurian, Orr's Creel-, 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 (/).

(d) Altered Silurian. In places micaceous, like (/); 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 (^), but more granitic. About

12 feet wide. (/) Metamorphic rock, schistose, and the joints lined with silvery mica. This

seems to embrace and overlie (g). (ff) Felstone vein or dyke, with minute patches of a foliated black mineral. 0i) Indurated slates and sandstones (Silurian), as seen down the course of Orr's

Creek (auriferous).. -(-&) Granitic vein crossing id), a few inches in width ; at the sides rather coarsely

crystalline-granular, of yellowish, orthoclase and translucent quartz. In

the centre the same, but coarser, and with aggregations of silvery 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 lying at an angle with beds which are in reality vertical, or nearly so.

As the granites of the series (b) have invaded, cut off, absorbed, or altered the Silurian strata, so also have extensive extravasations 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 crystalline-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 crystalline form of binary granite (Mitchell River) ; and, on the other hand, a highly porphyritic rock, 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