president's address — SECTION c. 137 



tinfield, which is situated on the head waters of the Turner River, 

 about 24 miles from Port Hedland. The district consists of a series 

 of metamorphic sedimentary and bedded igneous rocks skirting the 

 margin of an extensive granite mass, several hundred square miles in 

 extent. These sedimentary auriferous rocks are pierced by a multitude 

 of granite veins, which in this district are of considerable economic 

 importance by reason of the fact that they form the matrices of the tin 

 and tantalite for which Wodgina is noted. Near the tinfield is an 

 instructive section showing a much older intrusive granite rock, which 

 has been invaded by the much newer (though still old) tin-bearing 

 granite. These acidic veins are made up of a coarse-grained rock 

 composed of mica, quartz, felspar, and now and then tourmaline, and 

 may be described as pegmatite, using the term in the sense in which 

 it was applied by Delesse for any coarse-grained granitic rock contain- 

 ing mica, quartz, felspar, and tourmahne. 



No reference to these old granitic rocks would be complete without 

 some mention being made of the large ice-like quartz reefs, which stand 

 up to considerable altitudes above the surface like a wall, and which 

 can, in some cases, be traced across country, with more or less inter- 

 ruption for miles. They may be described as veins or dykes of pure 

 silica. In some of the mine workings in two fields veins of this character 

 are to be seen cutting across the auriferous quartz reefs. The question 

 as to the relationship of these acid intrusive dykes of the type just 

 mentioned and these quartz reefs has recently been attracting con- 

 siderable attention, and there is a growing tendency to associate some 

 quartz reefs genetically with pegmatitic granite veins. In the Cool- 

 gardie Goldfield quartz reefs are often intimately associated with acidic 

 dykes, and in some cases the latter gradually pass into pure quartz 

 at their extremities. These acid dykes can be seen to pass by im- 

 perceptible gradations into the main mass of the granite at Coolgardie. 

 Similar instances have been noticed in the Wodgina neighborhood, 

 and doubtless there are numerous other instances, but possibly they 

 may have been overlooked, and their significance unappreciated. 



Now, a granite mass during the process of cooling gives rise to 

 more acid pegmatite veins, and, by further elimination of the bases, 

 pure quartz veins may result. 



Various intermediate stages between granite and quartz veins have 

 been noticed in Western Australia, and it is more than likely that many 

 quartz veins are very probably intrusive rocks directly secreted from a 

 cooling granite magma. Many pegmatitic granite veins contain tour- 

 maline crystals as one of their essential constituents, and many pure 

 quartz veins in Western Australia contain tourmaline also. 



Up to the present time, however, no observations have been made 

 in this State with the object of discovering whether there are any of 

 those structural and mineral changes induced in the enclosing rocks 

 by the introduction of the quartz such as may often be observed con- 

 sequent upon the injection of granitic veins. Observations upon this 

 head, which are much to be desired, would have considerable scientific 

 and even more important economic value. 



