132 FORMER LAND CONNECTIONS. 



Several independent lines of reasoning lead to the opinion, 

 generally accepted, that the earth possesses a solid shell or crust 

 and a fairly solid heated core, but that the transitional zone is 

 plastic or at least semi-plastic in its nature. The depth down to 

 this substratum is uncertain, but probably ranges between about 

 fifteen and fifty miles, and, if the crust be excessively loaded at any 

 point, it will tend to sink down at that spot, while at the same 

 time some of the substratum becomes forced out from beneath tht 

 weighted area and raises the crust adjoining thereto. 



It is a truism that the land is gradually being worn down and 

 its waste deposited in the ocean; the land therefore grows lighter, 

 while on the contrary the ocean floor adjoining becomes heavier 

 The balance between the land and the oceanic areas being thus 

 upset, the former tends to rise and the floor of the latter to sink. 



Once established, this differential movement continues, and 

 would go on indefinitely, widening its sphere of operation, were 

 it not for the behaviour of the sub-stratum in being squeezed out 

 from below the part of the crust supporting the gradually subsid- 

 ing ocean floor and making its way into the base of the rising land- 

 mass. Such conditions are quite unstable ; the crust of the earth 

 is usually under considerable lateral pressure and is resting not on 

 a fixed foundation, but on a now slowly creeping sub-stratum. The 

 result is that the land portion becomes compressed into a series of 

 more or less parallel folds and elevated as mountain ranges, while 

 the sub-stratum beneath, being a potential eruptive magma, liqui- 

 fies with relief of pressure and bursts upwards through the strata, 

 either pouring out at the surface in the form of lava or remaining 

 below ground and consolidating as an intrusive rock. (See Fig 2.) 



Earth Foldings Affecting Gondwanaland. 



Now this is precisely what has happened in South Africa. The 

 land lay in the north of the Union, and, in the southern part of 

 the Cape, sediments had accumulated to a depth of about four 

 miles during the interval from the Devonian to the Triassic. The 

 deformation became so great finally that the crust collapsed and 

 was thrown into a multitude of sharp folds, huge thicknesses of 

 even such hard rocks as quartzite being puckered up in concertina 

 fashion and the arches being often overturned ; in this manner the 

 Southern Cape Ranges striking east and west originated. The 

 area inland, beyond the belt of folding, was flooded with basaltic 

 lava-flows, while the strata beneath became riddled by sheets and 

 dykes of dolerite ; to the south subsidences took place and the 

 waters of the ocean proceeded to make inroads upon the newly 

 rising lands. 



In this cataclysmic cycle South Africa did not stand alone. 

 The belt of collapse extended westwards, passed through the Argen- 

 tine with a trend more to the north-west, and continued into 

 Northern Chili and Bolivia. The pressure there folded strata iden- 

 tical lithologically and palaeontologically with those of the Cape 

 and left untilted the Glossopteris-bea.rhig beds of Uruguay and 

 Southern Brazil, just as in the Great Karroo; further these deposits 



