238 



sericitized. A great deal of granular qxiartz is present, with 

 clear irregular grains of microcline. Biotite is present in 

 small amount, much oxidized. The magnetites are octahedral 

 idioblasts, but contain poikiloblastically so much of the 

 groundmass that they are plainly secondary. There is little 

 to indicate whether this rock should be placed in the ortho- 

 or para-gneisses, except that such a peculiar arrangement 

 of magnetite would hardly be expected in a granite-derived 

 gneiss. There seems no reason to consider as other than a 

 different example of the same formation as the last-described 

 specimen. 



III. — Discussions of Observations. 



The basic dykes of Blinman include melaphyres, olivine- 

 diabases, ophitic- and granulitic-diabases, and gabbro-dia- 

 bases. They are all very considerably altered, and the vari- 

 ous types of alteration present many features of interest. 

 The occurrence of similar rocks as far afield as at Leigh 

 Creek and the Barrier Ranges suggests the wide extent of 

 the area of eruptions of the same basic magma. 



With regard to the age of these rocks, Mr. Howchin 

 observes : — "Two considerations seem to point in the direc- 

 tion that the volcanic activity belonged to a late stage of 

 the elevation of the dome" [the geo-pericline, in the centre of 

 which is Blinman], "and that the dykes were formed at no 

 great depth from the present surface, viz., the lava of the 

 supposed necks is often vesicular in structure ; and, secondly, 

 whilst the slate and other rocks which have been penetrated 

 and reduced to breccia by the intrusive dykes show contact 

 effects, they have undergone no secondary metamorphic 

 change in the mass which might have been expected to occur 

 had they been brecciated at considerable depths. "<8' This 

 may be fully admitted, yet they can scarcely be newer than 

 Palaeozoic. While it is by no means an exact method, the 

 age of an eruptive rock may be gauged from the extent and 

 manner of its alteration. In the rocks before us the pre- 

 dominant alterations are the formation of uralite or other 

 secondary amphiboles by alteration of the pyroxenes, of scap- 

 olite, epidote, and zoisite ; by alteration of the felspars. The 

 formation of epidote and uralite is usually a process of 

 rather deep-seated alteration, (5) and though declared by Van 

 Hise (10) to be possible at comparatively shallow depths, is 



(8) Aust. Assocn. for the Advancement of Science, vol. xi., p. 

 418. 



(9) See Rosenbusch Iddings, Micr. Physiog. of Rock - forming 

 Minerals. 



(10) Treatise on Metamorphiem. 



