138 GREAT SERPENTINE BELT OF NEW SOUTH WALES, iv., 



1. The Alhite-dolerite and Spilite Group. 



General features. — These rocks are by far the most abundant 

 of the Devonian igneous suite. Mineralogically and chemically, 

 they form a very well marked homogeneous group; and, though 

 the variety of textures developed is very great, they are con- 

 nected with one another by a complete series of intermediate 

 textures. At the acid extremity of the group there are passage- 

 rocks into the keratophyres. The mineral composition is simple. 

 Throughout the whole group, the predominant mineral is an 

 acid plagioclase. Augite occurs in amount varying from one- 

 fifth to more than one-half of the amount of the felspar. Ilmenite 

 or titaniferous magnetite is next in order of abundance, but 

 varies within wide limits, as does also the amount of apatite. 

 Quartz varies greatly in amount, and in the role that it plays : it 

 may occur in isolated grains, in interstitial mosaic, or in micro- 

 pegmatitic intergrowth with felspar. Rarely a small amount 

 of primary, brown hornblende is present. Pyrites is usually 

 present, though only to a small extent. No definite sign of 

 olivine is seen, even among the decomposition-products. Potassic 

 minerals, such as biotite and ortjioclase, are absent. The 

 feature which distinguishes the whole group from rocks, similar 

 in chemical composition and geological association in other parts 

 of the world, is the comparative freedom from decomposition. 

 Secondary minerals are present in most of the rocks, calcite, 

 chlorite, or epidote, but usually only in small amount. Com- 

 plete uralitisation is almost confined to the dynamically altered 

 rocks of the Woolomin Series. 



Chemical composition. — The chemical composition of this series 

 is exemplified by the table herewith, and may be contrasted 

 with the average composition of basalts as given by DalyO). 

 The chief feature of our rocks is their richness in soda, and, to 

 a lesser extent, in titanium, (Daly points out that the two often 

 go together) while the alumina, potash, and lime are low. The 

 total iron oxides and magnesia, and the silica of the rocks free 

 from quartz, are about the same as that of the average normal 

 dolerite, 



