MICROSCOPIC STRUCTURE OF SOME AUSTRALIAN ROCKS. 201 



with radial zeolites. Some of these radiate, not from the centre 

 outwards, but from one or more points on the circumference, 

 inwards. They show a black cross, whole or in part, under the 

 usual conditions. As in the other slices of this rock, there are 

 crystals that present some perfectly sharp faces one or more of 

 which are clearly corroded. Magnetite occurs in grains and 

 aggregates. Towards the centre of the slide there is a very 

 interesting augite with a line of inclusions parallel to its outer 

 edges. The outer zone, beyond the inclusions, is of a lighter 

 colour in plane light. The growth of the crystal was no doubt 

 interrupted for a time and then resumed. The more recent 

 addition is in optical continuity with the older central portions. 

 With crossed nicols, a point of considerable interest is revealed — 

 the crystal, in polarized light, is shown to be a remarkably fine 

 twin. The trace of the composition plane is a line traversing the 

 centre, and it will be noted that the newer accretions adapted 

 themselves to the condition of elasticity of the parts around which 

 they grew. In common light nothing of this structure is shown. 

 Slice 11. — Porphyritic Basalt, same locality as Slice 9. In 

 general characters it resembles the other slices from German's 

 Hill. Porphyritic plagioclases are plentiful, some being beautifully 

 twined. They show some broken and some corroded examples. 

 There are a few olivine pseudomorphs present. There is a twin 

 felspar on this slice cut nearly perpendicularly to the brachypina- 

 ■coid. The adjacent lamellae extinguish at an angle of 37° and 34° 

 respectively. Of course this might point to albite, oligoclase or 

 microline, but there are sufficient grounds for deciding that the 

 felspar is oligoclase. The study of these slices shows that the 

 micro-porphyritic crystals in these rocks are allogenic in origin 

 and were floated up probably from greater depths, and in this 

 way received the broken and corroded appearance they usually 

 present under the microscope. One other crystal on this slice is 

 suggestive of some facts recently dealt with by Professor J. W. 

 Judd.* There is little doubt left in the Writer's mind that in the 



* On the Growth of Crystals in Igneous Rocks after their Consolida- 

 tion — Quart. Journ. Geol. Soc, Vol. xlv., p. 175. 



