99 



crystals will always sink, for if tliey are deposited on the 

 magma chamber walls, they will be supported by them Just 

 as a heavy salt will crystallize on the side of a beaker, not 

 necessarily falling to the bottom. 



It is difficult to suggest a probable theory explaining the 

 origin of these nodules of Iherzolite. At Mount Gambler 

 the ash-beds, which are of considerable thickness, contain an 

 abundance of these nodules in irregular layers, together with 

 large fragments of limestone, dolomite, and sand. It seems 

 likely that, after the outflow of lava, a subsequent eruption 

 shattered the crust down to the hypothetical peripheral layer 

 of peridotite, and the ejectamenta was deposited on the top 

 of the olivine basalt. 



In his appendix to "Notes on the Volcanic History of 

 Mount Shadwell,"(i6) Victoria, by J. T. Jutson,(i7) A. Chap- 

 man, A.L.S., describes a volcanic bomb or nodule and an 

 olivine-bearing rock ; the former contains olivine, diopside, 

 and bronzite, and the latter essentially fayalite. 



W. H. Twelvetrees, F.G.S., and W. F. Petterd, 

 C.M.Z.S.,<is' figure and describe a Iherzolite near the Wara- 

 tah-Corinna Road, Hazlewood District, Tasmania. It occurs 

 in the form of an intrusive dyke, and contains olivine, ensta- 

 tite, and monoclinic pyroxene. 



There is a Iherzolite described and figured by Teall,(i9) 

 from Vicdessoo, in the Pyrenees, which approaches the one 

 described in this paper. It contains chrome-diopside and 

 green spinel, whereas the Mount Gambler type replaces the 

 former constituent with diallage. A further occurrence was 

 noted by A. Lacroix (20) from the tuff -beds in the Pyrenees. 

 The peridotite exists in the form of friable nodules or bombs. 

 The only point of difference between this peridotite and the 

 one described is that the diopside is replaced by diallage, and 

 the latter rock is much lighter in colour than the former. 

 Another type of peridotite is described from Halival,(2i) Isle 

 of Rum, containing olivine, a bright-green augite, probably 

 diopside, a pleochroic rhombic pyroxene, hypersthene, and 

 chromite and picotite. The rock is dark-brownish-green in 

 colour, unlike the Mount Gambler variety. 



(16) Mount Shadwell is situated in the Hampden District in 

 Western Victoria. 



(17) Victprian Naturalist, vol. xxii., No. 1, May, 1905, p. 8. 



(18) Papers and Proc. Roy. Soc, Tas., 1897. 



(19^ Teal], J. J. H., British Petrography, plate i., fig. i. 

 (20) Lacroix, A., Mineralogie de la France, p. 187. 



(2i)Judd, Prof. J. W., Tertiary and Older Peridotites of 

 Scotland, p. 392. 

 d2 



