MICEOSCOPIC STKUCTUKE OF SOME AUSTKALIAN ROCKS. 209 



No complete pseudomorphs occur, the alteration not having 

 proceeded beyond the cracks of the olivine crystals. 



Slice 36. — Intrusive basalt from Kiama, New South Wales. 

 This rock is intrusive in the Permo-Carboniferous rocks of Kiama. 

 Examined in hand specimens it is seen to be a dense basaltic- 

 looking rock very fine-grained in texture, with porphyritic crystals 

 of a striated felspar. These porphyritic masses sometimes measure 

 as much as a quarter of an inch across. Examined in thin slices 

 the rock is seen to consist of a felted mass of plagioclases, wedged 

 between which are granules of augite and de vitrified glass rendered 

 dense by magnetite dust. The lath-shaped plagioclases are 

 traversed by numerous, long, colourless acicular rays. Nothing 

 is known of the nature of these bodies, but they occur in similar 

 rocks all over the globe. Zirkel thinks they belong to some un- 

 determined product of devitrification.* They sometimes stand 

 parallel one with another, or are grouped in the shape of fasciculi 

 and bundles. The plagioclases are of two generations. The por- 

 phyritic felspars are of allogenic origin and have evidently floated 

 for a long time in a liquid magma. They are corroded to a very 

 notable extent, indeed. I could find no sections in the slide 

 suitable for measurement, with a view to determine the species. 

 They are however, probably identical with crystals of a smaller 

 stature that are also present in this slide. One of the latter is 

 twinned so as to extinguish at almost equal angles, on either side 

 of the trace of the composition plane. The extinction is inclined 

 35° to one side and 34° to the other. The felspar is, therefore, 

 under the circumstances, probably oligoclase. Magnetite is also 

 present in well-formed cubes. The pyroxene of the rock is also 

 porphyritic and of a rich yellow colour by transmitted light. I 

 have only had the opportunity of cutting two slices of this rock — 

 hardly sufficient material I think, on which to found a name. 

 For the present, therefore, we shall designate the rock an intrusive 

 basalt. The Slices 36 and 37 show all the characters to which I 

 have referred. 



* Zirkel's Microscopical Petrography of the Fortieth Parallel, p. 232. 

 N-O«tober7, 1891. 



