BY H. I. JENSEN. 571 



to the west. This line has therefore been a hinge in all the 

 earth-movements which have taken place since the Silurian. It 

 has escaped very heavy sedimentation. For these reasons it 

 became fractured — or pre-existing fractures widened — in the 

 great earth-movements of the early Tertiary, and as there was 

 no heavy load of tenacious sediments to block the upward passage 

 of magmas, even those of a highly viscous character were able to 

 reach the surface. 



Avery interesting physical feature of the Warrumbungle and 

 Nandewar districts is the very great abundance of powerful 

 springs in the mountains. These occur chiefly above the artesian 

 intake, at elevations of from 2000 to 4000 or 5000 feet above sea 

 level, and are especially abundant in the volcanic rock, or at the 

 edges of intrusions of it. In every part of Australia we find 

 springs abundant in areas of igneous rock and crystalline schist, 

 whereas over sedimentary areas, in the west at any rate, springs 

 are not met with. There is no doubt that the water of these 

 springs comes from very deep-seated sources. Why is it restricted 

 to certain areas 1 The answer seems to me to be that it escapes 

 in other parts as well, but is absorbed before reaching the surface 

 by the mantle of artesian and subartesian strata. Hence these 

 mountain-springs prove to my mind that Professor Gregory's 

 theory of the origin of artesian water is at least in part correct. 



I now propose to deal briefly with the possible mode of origin 

 of the alkaline rocks. 



In their mineral composition and chemical composition the 

 segirine trachytes of dark greasy green colour are intermediate 

 between the corundum basalts (W.40) and the arfvedsonite 

 trachytes (W-16, W.17, W.38). Hence the monzonose, being 

 also a more abundant rock-type, may be considered the parent 

 magma. 



Again, "The Spire" orthoclase basalt (W.67) is chemically 

 and mineralogically very near the mean between a typical 

 dolerite (such as N.17 from the Dingo Creek laccolite, Nandewar 

 Mountains) and a trachy-andesite; and there can be little doubt 

 that this rock owes its characters to a mixture of magmas. The 



