98 



The tachylytic coating consists essentially of a light- 

 brown glass crowded with magnetite grains, rendering it 

 almost opaque. A few partially-absorbed grains of olivine 

 and augite are scattered about with little or no felspar. 



From the above considerations the rock is evidently plu- 

 tonic and allied to the enstatite peridotites. 



With regard to the mode of origin of nodules such as 

 these, there appears much controversy in geological litera- 

 ture. 



R. A. Daly (i**) attributes the origin of a great number 

 of igneous rock types to a differentiation of a parent olivine 

 basalt. He points out that fractional crystallization is one 

 of the important factors in the formation of these rocks, and 

 that the phenocrysts of olivine, augite, and magnetite sink 

 in the magma to certain levels whilst in the conduit. Here 

 they may be redissolved, increasing the basicity in the lower, 

 hotter part of the lava column, which on crystallization pro- 

 duces peridotites, or, following extrusion, develops picritic and 

 limburgitic rocks. The probability is, however, that the oli- 

 vine nodules, occurring in the ash-beds at Mount Gambler, 

 have been formed in a similar way. At Mount Gambler 

 there appears a first stage, when olivine basalt was erupted, 

 and later a huge deposition of fragmentary material con- 

 taining the Iherzolite nodules, and then small eruptions of 

 slaggy and glassy lava. If this be the case, the injected ultra- 

 basic rock beneath, in the conduit, has been erupted, and 

 during its passage through the conduit has become coated and 

 caught up in the glassy basalt, and ejected at the surface 

 as fragments. However, in this theory it is difficult to ac- 

 count for the association of minerals with a large variation 

 in specific gravity. We have to account, not for the forma- 

 tion of a nodule containing olivine, enstatite, or picotite, but 

 of olivine, enstatite, and picotite, with specific gravities of 

 3'4, 32, and 4*l-4'5 respectively. 



Another view which probably accounts for the inclusions 

 in the basalt, and as isolated fragments coated with basalt, 

 is discussed by Lacroix.^^^) He holds that the basic minerals 

 separate out first from the magma, forming an ultra-basic 

 border zone, and, successively, more acid rock types are de- 

 veloped within. If just after the crystallization of the outer 

 crust eruption occurred, the basalt, less basic now than the 

 original magma, would contain ultra-basic inclusions only. 

 Moreover, it is not necessary to suppose that the heavy basic 



(1'''' Journal of Geology, vol. xvi., No. 5, Julv-August, 1908. 

 pp. 401-420. 



(i5)Lacroix, A., Les Endures des Roches Volcaniques. 



