president's address— section c. 79 



which underlaid the original crust, with the result that some 

 magma will be forced up, and a volcanic eruption \^'ill be 

 produced. 



If, however, the eruption be situated within the area of 

 heavy sedimentation the resulting lavas will usually be, not 

 the light acidic obsidians or rhyolites, but the heavy basalts, 

 owing to the displacement of the acid granitic magma from 

 under the area of sedimentation, owing to the downward 

 bulging of this loaded portion of the crust. 



For the same reason submarine eruptions usually produce 

 basalts owing to the great depression wliich the crust has 

 undergone under oceanic basins having led to the light 

 granite magma having worked its way from under them 

 towards continental areas where it is capable of rising under 

 the crust to a higher altitude than it can in submarine areas. 

 Thus the tendency probably is for granitic magma to leave 

 areas of subsidence or any low-lying portions of the earth's 

 crust and concentrate itself under the higlier portions of 

 continents in a manner analogous to the rock oil of the United 

 States, which, owing to its being of less density than water 

 when imprisoned in undulating water-bearing strata, with- 

 draws itself from the synclinal troughs and concentrates 

 itself along the arches of anticlines. This slow migration of 

 the granitic magma may be assisted, as pointed out by Mr, 

 Mellard Reade and Professor Hutton, by internal earth 

 tides, if such exist. 



In order that basic eruptions may be produced as a result of 

 sedimentation, it is not necessary to assume that the sediments 

 must attain a thickness of about twenty miles in order to 

 bring them down to the normal level of the isogeotherm of 

 the basalts. Volcanic eruptions, even when of a general 

 basic character, usually produce lavas of an intermediate 

 composition at their commencement, and afterwards basalts. 



This sequence of volcanic rocks, which has frequently 

 been observed in other portions of the world, is confirmed 

 by the evidence at present attainable in Australia. 



If these theories be now applied to afford a possible 

 explanation of the volcanic phenomena of Eastern Australia 

 and Tasmania discussed in this paper, it appears to the 

 author that the following conclusions may be tentatively 

 suggested. During the deposition of the 30,000 feet of 

 Upper and Lower Silurian sediment there is no evidence of 

 contemporaneous volcanic action. The Silurian rocks were 

 then plicated, uplifted, and partly denuded to form the lower 



