37 



The red aplite is possibly to be regarded as a step in the 

 differentiation of the granitic magma. It does not, however, 

 fit very well into the variation diagram, and it differs from 

 the other rocks (except the soda aplite) in containing no 

 ferro-magnesiah minerals. This fact rather negatives the idea 

 of the rock being one of a differentiation series, of which the 

 biotite-bearing syenite is an end member ; it suggests, indeed, 

 that the red aplite originated by some process distinct from 

 that which operated to produce the diorite-granite-syenit-e 

 series. 



It is to be noted that the red aplite and the dolerite 

 appear to stand in a complementary relationship towards the 

 granite. A combination of the analyses of the two rocks in 

 the proportions of 9 of aplite to 4 of dolerite gives a mean 

 result very comparable on the whole with that of the porphy- 

 ritic granite when we consider the rather peculiar composition 

 of the dolerite : — 



Weighted mean 

 of red aplite Porphyritic granite, 

 and dolerite. 



SiO. 



... 68-24 



68*20 



ALO, ... 



... 15-46 



15-99 



Fe;03 ... 



... 0-72 



0-89 



FeO 



... 2-78 



2-65 



MgO ... 



0'71 



0-80 



CaO 



3-63 



2-61 



Na,0 ... 



2-74 



2-85 



K3O 



... 4-42 



4*60 



TiO. 



... 0-68 



0*58 



Furthermore, the association of dolerites and quartz 

 dolerit^s with soda aplites and granophyres has been noticed 

 in different parts of the world, as for example at St. David 

 Head in Wales, (28) in the Gowganda District of Ontario, 

 Canada, (29) and elsewhere. 



As can be seen by the list of analyses (vide supra) these 

 soda aplites are quite comparable with that of Port Elliot, and 

 it is here suggested that the red aplite, the soda aplite and 

 granophyres, and the dolerites should be regarded as being 

 closely related to each other and to the granite. 



This suggestion of a connection between the dolerites and 

 the aplites, both sodic and potassic, is not without support 

 from other parts of South Australia. From various localities 

 dykes of uralitic dolerite have been described cutting Cambrian 



(28) J. V. Elsden: Q.J.G.S., vol. Ixiv., 1908, p. 273. 



(29) W. H. Collins: Geol. Surv. of Canada, Memoir 33, 1913. 



