522 THE DISTRIBUTION, ETC., OF ALKALINE ROCKS, 



quartzites, spinel-rocks, andalusite-rocks, cordierite-rocks, wollas- 

 tonite, and acid granites and gneisses rich in albite and orthoclase. 

 The earliest lavas would have been essentially calcium-aluminium 

 silicates which would now be represented by amphibolite, or if 

 they had assimilated much alkali by calcic granite: and also more 

 basic non-aluminous lavas from more deep-seated sources (peri- 

 dotite, serpentine, augitite, etc.). 



The earliest sediments would consist of chemical precipitates 

 (carbonates of alkaline earths and chlorides of the alkalies princi- 

 pally), and detritus washed from the earliest lands. These bedded 

 rocks would consist of limestone, dolomite, iron ores, salt beds, 

 grits, conglomerates and shales, arkoses, tuffs and breccias. 



Now these are the rocks whose metamorphic representatives 

 we actually do find in the deepest Archaean; confused and mixed 

 up with a network of intrusions of igneous rock, we get marbles, 

 dolomites, iron ores, jadeite, conglomerate schist and gneiss, mica- 

 schists, gneisses, and the metamorphosed representatives of tuffs 

 and breccias (amphibolites and glaucophanites) in our fundamental 

 complex. 



The alkaline primordial sediments and precipitates became 

 more and more depressed under the weight of Palaeozoic and 

 Mesozoic sediments. 



Being easily fusible and hydrous, heat and pressure might 

 easily liquefy them, or, at all events, turn them into a viscous 

 fluid, when they would slowly move laterally towards regions of 

 smaller pressure, namely, towards continental areas and particu- 

 larly towards those parts where mighty faults had further dimin- 

 ished pressure by establishing communication with the surface. 

 In the trough-faulting of such fractured continents subsiding 

 blocks become squeezed between the standing segments and also 

 exert great pressure upon subjacent magmas. In this way the 

 alkaline viscous magma, now rendered siliceous and impure by 

 chemical reactions with the rocks met with on its travels, would 

 be squeezed out along the fault-planes. 



The Eocene, being a period of immense crustal readjustments, 

 produced favourable conditions for the expulsion of the magma. 



