MICROSCOPIC STRUCTURE OF SOME AUSTRALIAN ROCKS. 191 



the lath-shaped crystals are seldom complete at their ends. They 

 branch and have step-like terminations, but perfect sections are 

 rare. On the contrary, the more basic minerals as olivine and 

 augite are commonly idiomorphic. 



The slices of the basic rocks show examples of the structures 

 referred to by Rosenbusch under the headings of hyalopilitic, 

 pilo-taxitic, hypo-crystalline-porphyritic, vitro-porphyritic, and 

 holocrystalline-porphyritic* 



In the order of consolidation our basalts generally follow the 

 law of decreasing basicity, and in this respect correspond with 

 similar rocks from other parts of the globe. A possible exception 

 may exist in cases where it will be pointed out that in certain 

 eruptive rocks the felspar has preceded the augite, while in others 

 the augite has preceded the felspar. Instances of this kind are 

 of particular interest in view of the attention which the subject 

 is receiving at this time. I may state that the commonly accepted 

 views on the subject are clearly summarised by Mr. J. J. Harris 

 Teal in a lecture delivered in the Woodwardian Museum, Cam- 

 bridge.! Professor J. W. Judd points out that other influences, 

 besides an increasing acidic magma, may determine the order of 

 solidification, particularly as interpreted by enclosures of one 

 mineral in another. J Those who consider, he remarks, the suc- 

 cessive crops of crystals formed in a rock, as being determined by 

 the alteration in the composition of the residual magma by the 

 constant separation of the basic minerals from it, will find it 

 difficult to harmonize such views with the facts presented to us at 

 Krakatoa. The rocks of Krakatoa strongly suggest, that the 

 minerals crystallized out of a magma, and the residuum of mixed 

 silicates can be separated to a greater or lesser extent and then 

 recombined in fresh proportions. 



* Rosenbusch, Mikroskopische Physiographie der Massigen Gresteine, 

 Second Edition, pp. 726 - 728. 



t Nature, March 12th, 1885, p. 447. 



X The Lavas of Krakatoa, by Professor J. W. Judd — Geol. Mag. Dec. 

 III., Vol. v., pp. 8-9. 



