Dyke Stones . 
113 
CHAPTER XI. 
Dyke Stones . — Various Geological Ages. 1st Group , iVbw- 
auriferous ; auriferous. 2nd Group. 
Dykes of igneous rocks showing an infinite variety of mineral 
structure and composition intersect the Palaeozoic and Mesozoic 
rocks in the form of injected wall-like masses of from a few 
inches to hundreds of feet in thickness, and are sometimes 
traceable for many miles, either continuously or by disconnected 
outcrops in some defined bearing. Some of these dykes are 
evidently products of Plutonic action during Palaeozoic times, 
while others have been injected since the Mesozoic period, and 
there is a marked difference of mineral composition between the 
dyko stones of the two periods. The first group comprises 
numerous varieties of granite, syenite, quartz -porphyry, felsite- 
porphyry (elvanite), gabbro, greenstone, diorite, &c. ; while the 
second or more recent class consists of rocks of a basaltic 
character, anamesite, anamesite-porphyry, basalt, lava, &c. 
1st group. — Intersecting the granite masses throughout the 
colony are numerous dykes of very fine-grained granite, binary 
or half-granite (aplito), euritej and occasionally schorlaceoua 
granite. Granite, half- granite (aplite), syenite-granite, ternary and 
quaternary, granite-porphyry, syenite-granite-porphyry, felspar - 
porphyrite, felsite,folsite-porphyry, (elvanite), and quartz-porphyry 
in an infinite variety of composition and texture occur as dykes 
traversing granite, metamorphic Lower and Upper Silurian rocks. 
In different portions of any one dyko considerable diversities of 
composition and texture may be found. Detailed descriptions by 
Professor Ulrich of typical specimens of these rocks will be found 
in the descriptive catalogue of the Industrial and Technological 
Museum. 
These dykes do not appear to possess other than scientific 
interest, not being, as far as has yet been ascertained, in any marked 
degree connected with the auriferous character of the quartz- 
veins in the rocks which they intersect. According to Professor 
Ulrich’s descriptions of the above-named varieties the predominat- 
ing felspar in all of them is orthoelase, and though hornblende 
occurs sometimes as an accessory mineral, and contributes the 
syenitic character, it is not a prevailing ingredient, and frequently 
occurs only in certain portions of dykes which elsewhere do not 
contain it. 
