THi: Dol.KlMTKs <)K Kl\.; GEORGE I. VXD VXD ADK.L1K LAND I'.HOWNK. 249 

 The pyroxene is evenly distributed through the rock in typical ophitic relation- 



ship ti. the felspar. (See hate XXXYI1I. Fig. 1.) 



The specimen Nn. 732A was taken from the debii* at the foot of the Organ Pipes, 

 and is eoarser than No. 732B. It is a greyish-black rock in hand-specimen, with 

 occasional pyroxene prisms visible up to 3 mm. in length. The principal constituents 

 are. in volume percentages : 



Plagioclase ............ 54 



Pyroxene ............... 33 



Mesostasis ............ 11 



Iron Ores ............ 2 



This constitution corresponds pretty closely with the norm of the rock, given below. 

 and indicates that the rock has a normative mode. 



I'lagioclase is in stumpy prisms up to -5 mm., though a few larger individuals 

 attain a length of 1-5 mm. For the most part the prisms only dent the outlines of the 

 pyroxenes, which are often subidiomorphic, so that typical ophitic fabric is absent. 

 Parallel intergrowths of pyroxenes with different optic axial angles are frequent. There 

 is an absence of rhombic pyroxene, biotite, and the green and brown pseudomorphs so 

 characteristic of No. 732B. {See Plate XXXVIII, Fig. 2.) 



The third of this series (No. 733) is coarser in grain than the other two, and differs 

 much from them in other respects. It is a brownish-coloured rock with an average 

 grainsize of about 4 or 5 mm. The dominant characteristic is the presence of many 

 flashing cleavage-planes of pyroxene, slightly curved at times and exhibiting quite a 

 bronzy lustre. 



The minerals present are : 



Plagioclase ............ 33 per cent, by volume. 



Pyroxene ............ 27 ,, 



Iron Ore ............ 2 ,, 



...... ...... 38 ,, 



The plagioclase forms thick prisms varying in length up to about li mm. and giving 

 almost >< plan 1 sect ions at right angles to (001) and (010). There are also smaller prisms. 

 some of microlitic dimensions. The felspars often show magmatic resorption and 

 embayment of their edges just like the phenocrysts of a porphyritic rock, and inclusions 

 of the mesostasis are fairly frequent. Indeed the mesostasis is so abundant that it 

 ives rise in places to what Iddings would have called porphyritic intersertal fabric, 

 the pyroxene and felspar playing the part of abundant phenocrysts. 



The pyroxene is all monoclinic. and. judging by the si/.e of the optic axial angle 

 where it can \>e observed, is mostly, at all events, enstatite-augite. The salite 

 striation is extremely common, and to a less extent the diallage striation, both accom- 

 panied by a brownish decomposition product which gives rise to the bronzy lustre 



