VOLCANIC NECK AT THE BASIN, NEPEAN KIVER. 145 



Dr. L. A. Ootton and Prof. W. N. Benson on various points, 

 and through the kindness of the latter was able to examine 

 bis slides of the Dundas rocks. 



To Mr. G. W. Card, a.r.s.m., f.g.s., he acknowledges 

 many kindnesses, and the opportunity of examining material 

 belonging to the Geological Survey. And in particular to 

 Mr. \V. R. Browne, B.Sc , for help in petrographical difficul- 

 ties, and for advice generally, his best thanks are due. 

 Finally he wishes to express gratitude to Mr. W. E. 

 Baines, j.p., a resident of the Basin district, for much 

 kindness extended to him during his visits to the Basin 

 area. 



Explanation op Plate VII. 



Fig. 1. Troctolite, showing relations between felspar and olivine, 

 the latter exhibiting initial alteration. Polarised light x 15. 



Fig. 2. Crystal of Hypersthene in norite showing the development 

 of " Schiller " inclusions parallel to three planes. Polarised 

 light x 22. 



Fig. 3. Granophyric intergrowth between green spinel and augite 

 in troctolite. Ordinary light x 86 (high power). 



Fig. 4. Graphic intergrowth of picotite and pyroxene in harz- 

 burgite. Ordinary light x 31. 



Fig. 5. Common mode of decomposition after olivine in lherzolite, 

 the products comprising bowlingite = b, serpentine = s, quartz 

 (chalcedonic?) = q, and an indeterminate brown substance, 

 the last two being always directly associated. Polarised 

 light x22. 



Fig. 6. Section across junction of a gabbroid xenolith and its host 

 (basalt) showing the manner in which felspar is more readily 

 assimilated than pyroxene by the enclosing magma. Polarised 

 light x 22. 



J— September 1, 1920. 



