ANORTHOCLASE BASALT FROM MiiPLETON, QUEENSLAND. 167 



PETROGENIC SIGNIFICANCE. 



In a previous publication^ the author, in describing 

 the Spieer's Peak oligoclase basalt, fully recognised its 

 chemical difference from the ordinary sub-alkaline flows of 

 Southern Q:ueensland, and showed the close chemical rela- 

 tionship with that of the mugearites described by Harker** 

 from Skye. Similar chemical characters were noted in the 

 oligoclase basalt and the olivine basalt from ^Mount Lindsay, 

 both of which terminated phases of effusion of lavas, the 

 lower and upper basaltic series respectively, and the author 

 Avrote' : "Whether the occurrence of these rocks at the 

 termination of two periods of activity during which basic 

 rocks have been poured out is a mere coincidence or not, 

 is a question. " It is a matter of very considerable interest 

 to find that this alkaline basalt from ]\Iapleton, which has 

 the same peculiar chemical characters as the basalts from 

 Spieer's Peak and from Blount Lindsay, like them also 

 represents the concluding effusion of a volcanic phase — the 

 Upper Basaltic one. 



One would expect the concluding effusions of each of 

 these phases to be more differentiated than the earlier flows, 

 owing to' the law of increasing divergence, but the alkaline 

 character of the last flow in each case has a particular 

 interest and certainly does not support Daly's assimilation 

 hj'pothesis as to the origin of alkaline rocks. 



In conclusion, I extend to jMiss Rose Scott. M.Sc, and 

 ]Mr. G. J. Saunders, B.E., ]M.Sc., my thanks for carrying out, 

 when advanced students at the University, the two analyses 

 which have hitherto been unpublished, and which have been 

 of the greatest help in the points considered in this paper. 



^ ' ' Volcanic Rocks of South-Eastern Queensland, ' ' Pr. Eoy. Soc 

 Qld., xxvii., 1916, p. 172, p. 176. 



'■ Ilarkei", ' ' Tertiary Igneous Eocks of Skye, " ' p. 263. 

 ' Op. Cit., p. 192. 



