ROCK SPECIMENS FROM CENTRAL AND WESTERN AUSTRALIA. 313 



it does not hold good for Western Australia, where alkaline 

 rocks are quite unknown, nor to New Zealand, where the 

 same alkaline series as that of Erebus is well displayed, 

 but where quartz-dolerites are so far unknown. 



In opposition to the suggestion of a petrographical magma 

 common to the whole of Gondwana Land, it may be urged 

 that the class of rocks relied on, viz. quartz-dolerites with 

 hypersthene or enstatite-augite are not by any means con- 

 fined to remains of that ancient continent, but are equally 

 common in British Guiana, Great Britain, Canada, etc. 

 That would be to imply, however, that similar petrographical 

 provinces could not exist in different parts of the earth. A 

 stronger objection is furnished by certain theoretical views 

 on the mode of formation of these rocks. It has been 

 suggested by Daly 1 that quartz-gabbros and the commonly 

 associated granophyres are formed by the acidification of 

 gabbros by the assimilation of the surrounding walls. 

 Tyrell 2 has elaborated this view with special regard to 

 quartz-dolerites, and suggests further that they represent 

 a critical stage in assimilation, in that they are almost 

 entirely made up of intergrowths of related minerals, and 

 have reached the limit of saturation of a basic magma with 

 quartz. Both these writers totally fail to explain the 

 excess of magnesia and iron over non-felspathisable lime 

 which is necessary for the formation of hypersthene or 

 enstatite-augite. Whether we admit with them that quartz- 

 dolerites have arisen by the assimilation of acid material 

 by a basic magma, or agree with Vogt that they have 

 arisen by differentiation as an "anchi-eutectic" rock, we 

 must still postulate that the primary magma had funda- 



1 Daly, R. A., The Secondary Origin of Certain Granites, Am. Journ. 

 Sci., xx, 1905, pp. 185 - 216. 



2 Tyrell, G. W., Geology and Petrology of the Intrusions of the 

 Kilsyth-Croy District, Dumbartonshire. Geol. Mag., Dec. 5, Vol. vn, 

 1909, pp. 299 - 309 and 359 - 366. 



