Quartz in Basalt. liJl 



(1) Natural History of Igneous Rocks, p. 322. A. Harker. 



(m) Data of Geo-chemistry. F. W. Clarke. 1911. 



(n) Petrology of the Kalgoorlie Goldfield. J. A. Thomson. 



Geol. Mag., Vol. LXTX., 1913. 

 (o) Geology of Kalgoorlie. C. 0. G. Larcombe. Proc. Aust. 



I.M.E., Vol. V., No. II. 



IX.— Final considerations. 



The efforts to account for the presence of quartz in basic igneous 

 rocks have been many : — 



1. Iddings believes that at great depths, and under the mineral- 

 ising influence of water, in the case of great pressures, quartz could 

 crystallise out from a basic magma, and while in most cases the 

 quartz would later be entirely resorbed, the quartz basalts represent 

 cases where the resorption is incomplete. This theory, while it 

 would satisfactorily explain most features of the case, takes us 

 deep into the region of practically unknown physical properties 

 and processes. Iddings suggests that the occurrence of free quartz 

 in basic rocks is analogous with the occurrence of iron-olivine in 

 acid rocks. 



2. Daly suggests, in accordance with his theory of a fundamental 

 basalt magma, from which all igneous rocks are derived — that the 

 quartz found in basalts represents part of the overlying lighter 

 siliceous layer caught up and not fully assimilated by the basalt. 

 This fascinating generalisation would easily lend itself to an 

 explanation of all quartz in basic rocks, but from its fundamental 

 nature it is a question the discussion of which must be left to experi 

 petrologists. 



3. The most coinmon explanation advanced is that the quartz 

 is derived from acid rocks through which the basalt has passed oil 

 its way to the surface. 



As far as is known, the country rock at Mt. Greenock is wholly 

 Ordovician sediments; these are all tine grained, and contain no 

 beds from which the quartz could be derived. If the latter mineral 

 came from the very numerous quartz veins that traverse the ordovi- 

 cian sediments — and from its microscopic nature this is quite pos- 

 sible — we should also expect some fragments of the slates them- 

 selves to be still undigested. Close search has failed to reveal any 

 trace of a slate inclusion, a fact which seems sufficient to invalidate 

 that theory. 



4. It is also suggested that the quartz was picked up by the 

 lava at the surface. 



10a 



