HOW MOUNTAINS ARE FORMED — LYTTLETON 353 



the time (and as an additional flourish would make the continents 

 drift), and the readjustment when the number of cells increased is 

 associated in the theory with a period of intense mountain building. 

 Ingenious as this descriptive theory is, it is hard to see why hori- 

 zontal currents near the surface should produce such enormous uplifts, 

 and it is even more difficult to see why such currents should occur in 

 solid matter of considerable strength. It is difficult to prove one way 

 or the other whether a sufficient force will cause "solid" material to 

 flow if applied long enough, but there is recent evidence from the 

 motions of artificial satellites that the Earth may possess enough 

 strength to maintain a slightly more spheroidal form than its present 

 rotation warrants (presmnably a relic of a time of faster rotation). 

 This could tell heavily against the notion of convection currents. The 

 mechanism would also require regions of unequal heating to produce 

 currents, or some other departure from symmetry. Moreover, the 

 theory requires a growing core, and for this the theory speculates still 

 further and assumes that free iron is present deep within tlie solid 

 mantle. Because this iron would be heavier than the surroundings, 

 it would sink gradually to build up a metallic core. The Earth does 

 in fact contain a heavy core, almost entirely liquid, with radius now 

 some 55 percent of the whole Earth-radius, but the presence of the 

 requisite chunks of iron is highly dubious. A body containing nearly 

 40 percent of heavy metals would be a cosmic object of the utmost 

 curiosity. 



THE EARTH'S ORIGIN 



Several epochs of mountain building have now been traced right back 

 in time by the geologists. There have been at least three major periods 

 well authenticated in post-Cambrian times (that is, within the last 

 600 million years). Numerous others occurred over a range of some 

 thousands of millions of years, with their greatest intensity at intervals 

 of the order of a hundred million years. Thus any inquiry as to the 

 origin of mountains must face the question of the original state of 

 the Earth. It seems to be here tliat a new approach may bring 

 order where for so long there has seemed to be only difficulty 

 and contradiction. 



For almost a century it has been widely believed that the Earth 

 began its existence as an entirely molten body, so that its development 

 seemed to be explicable simply by the processes of cooling of such a 

 body. Indeed, the thermal-contraction hypothesis, whereby the moun- 

 tains are supposed to result from this cooling as it extends downward, 

 has long been regarded as the obvious cause. The surface would cool 

 first and become solid to a certain depth, and then, when a lower layer 

 cooled and contracted, the already solid outer crust would find itself 

 too large to fit continuously over the cooled adjacent interior. It 



