106 



KNOWLEDGE 



[June 1, 1891. 



coating ; but this doctrine of a tbin crust bas been aban- 

 doned on account of strong e\'idence telling against it. It 

 is quite possible for tbo eartb's interior to be bigbly heated 

 and yet to remain in tbo solid state. The enormous pres- 

 sure to wliicli rocks lying below the earth's crust (a word 

 still retained for convenience sake by geologists, but with- 

 out at all iiiiiilying a fluid state below) must be subjected 

 may be quite sufKcient to prevent their fusion, for pressure 

 probably retards the melting of solid bodies as it does the 

 converting into vapour of liquids. Hir 'William Thomson 

 proved that if any large portion of the earth were liquid, 

 or viscid, or even as elastic as a sphere of steel, it would 

 rise and fall as the ocean does under the influence of the 

 sun and moon, and thus produce tides which would con- 

 siderably interfere with the tide of the ocean as we now 

 observe it ; for it is clear that, if the land rose and fell at 

 the same time as the ocean, the one would obscure the 

 other. There is, possibly, a slight tide in the earth, but 

 only such as would be caused in a very non-elastic body ; 

 whereas with a fluid interior the crust would rise up and 

 down considerably, and would partly hide or very consider- 

 ably interfere with the tide we perceive in the ocean. 

 Some geologists maintani that there may be a great solid 

 nucleus, with a thin liquid layer over it, and a solid outer 

 layer or crust ; but there are some difficulties in the way 

 of this theory which cannot be considered here. 



Let us assume, however, that the lower regions of the 

 earth are in a highly heated state. Calculating from the 

 observed rate of increase of temperature on descending, it 

 is concluded that at a depth of 50 miles below the surface 

 there may be a temperature of 2000° C— sufficient to melt 

 even platinum (at the surface) ; and, further, let us sup- 

 pose such a temperature to be due to the earth having once 

 been in a molten condition (according to the nebular 

 theory). We have got, then, our source of heat ; let us 

 see what use we can make of it. Some geologists would 

 explain volcanic manifestations in this way. Vast quan- 

 tities of sedimentary matter slowly accumulate on the 

 floors of seas, in time forming great deposits of stratified 

 rock, thousands of feet thick ; as the materials accumulate 

 the bed of the sea slowly sinks under the weight of all this 

 material. The accumulation of so much rocky matter 

 checks the flow of the earth's heat, and so raises the 

 temperature of that region, thus acting as a bandage 

 does on the human body. Such sedimentary deposits 

 necessarily contain much water — probably as much 

 as from 5 to 20 per cent, of their mass. As the 

 strata sink, and as the earth's heat rises up to them, 

 their imprisoned water may be raised to a temperature far 

 above its ordinary boiling point. Streams of molten rock 

 penetrate thek mass, and generate more steam, or at least 

 highly heated water, and in this way. possibly a large 

 aniouMt of the sedimentary material nnght be nielted into 

 lava, and such lava, with its occluded steam, would be sure | 

 to find its way up through any lines of weakness in the 

 overlying rocks (such as fissures, faults, &c.), and on 

 reaching the surface would give rise to volcanic action. 

 There is a good deal to be said in favour of this explanation, ! 

 for it accounts for the steam which plays so important a ', 

 part. But it leaves out of sight some other aspects of the ', 

 question. Others aecoimted for the steam by the observed 

 proximity of active Volcanos to bodies of "water. They ' 

 argued that the water penetrating by infiltration through 

 the floor of the sea, found its way to highly heated 

 masses of rock, and was converted into steam, which 

 would find its way up through any fissures which the 

 stratified rocks above might contain. This theory seems 

 to harmonize well with some of the facts, but after what 

 bas been said it will not be necessary to go out of our way 



to account for the presence of the steam. Some geologists 

 are inclined to believe that here and there, below the 

 earth's sin-faee, there may be local reservoirs of still 

 molten matter, and that these are the supplies from which 

 the lava flows come, but this idea fails to account for the 

 association of \'olcanos with nuiuniain chains, and with 

 seas, as well as other imixirtant facts. A much more 

 promising explanation is that there are below the crust of 

 the earth large masses of highly heated rock, h/it solid by 

 the enormous pressure of overlying rocks ; but that when 

 earth-movements taking place within the crust, such as 

 the upheaving of mountain chains, take ofi' some of the 

 weight, the balance of pressure is no longer maintained, 

 and so these highly heated rock masses run off into the 

 liquid state, and find their way to the surface, producing 

 volcanic action. There is much to be said in favour of 

 this view. It rightly connects volcanic action with move- 

 ments of upheaval, with mountain chains, and lines of 

 weakness in the earth's crust. 



Now, the rocks in mountain chains arc found to have 

 suffered considerably from the eft'ects of heat and pressure, 

 and are seen to be considerably altered from their original 

 state. Shales and sandstones have been converted into 

 slates and quartzites, or even into schists and gneiss, and 

 in some eases into granite; so that metamorphism, as it is 

 called, seems to be closely connected with the upheaval of 

 mountain chains, the crumpling and folding of rocks, 

 which is very conspicuous in mountains, and with outbursts 

 of volcanic energy. 



The late Mr. Louis Mallet put forward a very ingenious 

 speculation which seemed to explain all those coincidence.s. 

 He endeavoured to show that the earth's supposed internal 

 heat might be dispensed with altogether, and that the 

 earth-movements themselves would be able to produce all 

 the heat required for volcanic action. Friction, we know, 

 produces heat, and he argued that the enormous friction 

 involved in the crumpling, crushing, folding, and displacing 

 of large masses of rock would be sufficient to bring about 

 the melting of portions here and there, and that volcanic 

 action would follow wherever these reached the surface. 

 The theory has attracted much attention, and was very ably 

 worked out ; but his experiments on the amount of heat 

 developed by the crushing of small portions of rock were 

 not quite satisfactory. That some amount of meta- 

 morphism is due to the heat produced by friction, and the 

 crushing of rock during earth-movements may be readily 

 granted, but it would be rash to assume more than this. 

 We do not yet know at what rate these Titanic forces of 

 mountain building work, and so any speculations as to the 

 temperatures developed by friction must be rather vague. 

 The theory, however, did good service in attempting to 

 explain the close connection between phenomena which 

 had previously been looked upon as in no way connected 

 with each other. Its simplicity and comprehensiveness 

 were much in its favour. 



But it seems like going out of our way to find a source 

 of heat. If the eartli is still a cooling body, wc have only 

 to fall back upon its own supply of heat, and to suppose 

 that somewhere not very far below the rocks forming its 

 surface there is a zone of highly heated rock, possibly in a 

 viscous state owing to pressure. As the earth slowly 

 loses heat, contraction of the outer shell follows, and this 

 must involve tangential strains and upheaving along certain 

 lines, because it has to settle down on to a smaller surface. 

 Thus continents are slowly raised, and mountain chains 

 upheaved. These movements cause changes in pressure, 

 and we can readily conceive that a change of pressure may 

 bring about the displacement and wedging up of matter 

 from below, which gets more and more fluid as it ascends 



