172 THE GEOLOG^i: OF THE CANOBOLAS MOUNTAINS, 



Petrological Descriptions. 



The petrology of the Silurian and Devonian rocks to the west 

 of the Canobolas, has ah'eady been described by one of us (C. A. 

 Sussmilch,* F.G.S.). The rocks probably underlie the later 

 volcanic series of the Canobolas, but, as they do not outcrop on 

 the surface in the area now under special consideration, no 

 further mention of them will be made here. 



The volcanic series is divisible into : — 



A. LeuGocratic Trachytes (including comendites, pantellarites, 

 light-coloured arfvedsonite-trachytes, etc.) and the dyke-rocks 

 corresponding to them. 



B. Melanocratic Trachytes (including phonolitic trachytes and 

 trachy-andesites), and the corresponding dyke-rocks. 



c. Andesites, chiefly of an alkaline facies. 



B. Basalts (with melilite and analcite.) 



With the exception of some of the basalts, all the rocks of this 

 volcanic series belong to the alkaline division. 



The plan will here be followed of describing a few of the typical 

 rocks of each group in some detail, and appending brief petro- 

 logical sketches of other varieties belonging to each group. 



Volcanic Sequence. — The earliest eruptions gave rise to flows, 

 plugs, necks, and dykes of leucocratic trachyte. 



The next eruptions yielded melanocratic trachytes and trachy- 

 andesites, and intrusions of their dyke-equivalents took place. 

 Pyroclastic rocks of this period are common. 



The following eruptions gave rise to alkaline andesites, and 

 the basalts were extruded last. The sequence is therefore one of 

 increasing basicity. 



It is also a matter of interest that the earliest eruptions were 

 of a very explosive nature, whereas the late eruptions poured out 

 immense quantities of lava in a quiet and peaceful manner. 



A. The Leucocratic Trachytes. — No.X.608. Field-name, Quartz- 

 Trachyte. Loc: Cadiangelong Creek. 



* Journ. Proc. Roy. Soc. N. S. Wales, Vol. xl., p. 130. 



