239 



rarely the effect of normal atmospheric weathering. Scap- 

 oiitization of felspar is almost always a deep-seated process. 



It would be well to briefly glance at rocks the age and his- 

 tory of which are well known to discover in what way these 

 microscopical criteria may be applied to determine the age 

 of basaltic and diabasic rocks. In the Tertiary igneous rocks 

 .of Skye,'i^> Harker does not appear to have found uralitiza- 

 tion of pyroxene except where the basalts are altered by 

 contact with gabbro. Epidote also occurs mostly in this situ- 

 ation. 



The alteration of pyroxene to amphibole is also often 

 observed in the Carboniferous basic eruptives, as, e..g., in 

 those of Arthur's Seat, near Edinburgh. (12) if ^-e turn to 

 Australian occurrences, there is not (to my knowledge) any 

 instance of changes similar to those undergone by the Blin- 

 man rocks in the Tertiary basalts of New South Wales ; but 

 there are frequent examples of uralitization and formation 

 of epidote in the Palaeozoic andesites.(i3) In the Mesozoic 

 diabases of Tasmania there is not such metamorphism as 

 is shown by the Blinman rocks. ^1^) In Victoria there is much 

 evidence to confirm this view. In several papers Howitt has 

 described Palaeozoic diabases and basalts, and clearly distin- 

 guishes the Devonian basalts from the Tertiary basalts, by 

 their type of alteration, chlorite, chalcedony, and carbonates 

 being typical of the Tertiary, epidote of the Palagozoic.^i^' 

 The Tertiary type of alteration by weathering may, of course, 

 be superimposed on the Palaeozoic. To some extent also 

 the type of alteration of the Heathcote and related diabases 

 resembles that of Blinman in the abundance of actinolite 

 and epidote, though there is no analogy to the Heathcote 

 chalcedonic diabases, (i^) These rocks are declared by Pro- 

 fessor Skeats to be Lower Ordovician.(i'^) 



Turning now to the rocks of South Australia that are 

 in any way comparable with the Blinman rocks, we again find 



(11) Geol. West-Oentral Skye with Soav. Harker. and Clough, 

 Mem. Geol. Sar. of Scotland, 1904, p. 28. (The larger memoir, '"The 

 Tertiary Igneous Rocks of Skye," was not obtainable at time of 

 writing.) 



(12) S. Allport Q.J.G.S., 1874; and many writers subsequently. 



(13) Numerous authorities might be cited here, .chiefly Notes 

 by G. W. Card and "W. A. Anderson. 



(14) Twelvetrees and Petterd, Proc. Roy. Soc, Tas. 1898-9, 

 p. 47. 



(15) Prog. Reports of Geol. Survey of Victoria, iv., 1874, p. 97. 



(16) Howitt, Notes' on Diabase and adjacent formations of 

 Heathcote district; Special Rep. Dep. Mines,, Victoria, 1896. 



(17) Roy. Soc, Victoria, xxi., N.S., pt. 1, 1908. 



