VOLCANIC NECK AT THE BASIN, NEPEAN RIVEK. 129 



ilmenite and felspar of the composition of labradorite, 

 determined by reading the angle between the pericline 

 twinning striation and tlie trace of the basal cleavage on 

 sections parallel to (010). 



The olivine is altered to the reddish mineral iddingsite, 

 and the felspars are sericitised in places. This rock is 

 interesting in being a linking type between hypersthene 

 gabbro, normal gabbro and olivine gabbro. • 



(c) Troctolitcs. — One of the most interesting features of 

 the work on the Basin material, is the recognition of the 

 presence of the olivine-felspar rock, troctolite, among the 

 inclusions. As far as the writer knows, this is the first 

 time troctolite has been recorded in New South Wales, and 

 although the occurrence is not as an individual rock mass, 

 still it has been deemed of sufficient importance to merit 

 an analysis. 



In hand specimens the troctolites show the typical 

 spotted appearance produced by the uniform grainsize of 

 the light felspar and dark olivine, characteristic of the 

 Forellenstein of von Lasaulx. A typical portion of the 

 field as seen under the microscope, is shown in Fig. 1, 

 Plate VIT. Microscopically the rock is seen to consist of 

 olivine in corroded grains, augite, felspar, a little ilmenite 

 and a granophyric intergrowth of pyroxene and green spinel. 

 The general features of the crystallisation are similar to 

 those of many British Tertiary ultrabasic rocks. Olivine 

 evidently commenced to crystallise before felspar, as the 

 former is sensibly corroded; there is no trace of eutectic 

 (not graphic) consolidation, and although the proportions 

 of the constituents are as follows: — felspar 60%, olivine30%, 

 augite 7%, spinel 1%, ilmenite 2%, still olivine has crystal- 

 lised first and yet could not originally have been in excess. 

 This, therefore, is evidently another example to be ex- 

 plained on the hypothesis which Vogt has assumed, that 



I— September 1, i£20. 



