ISO HISTORY OF GEOLOGY AND PALEONTOLOGY. 



great pressure was exerted by atmospheric vapours of water, 

 and the molten material became saturated with these. 

 Chemical processes took place, and gradually a firm crust 

 formed. The lower layers of this crust came by degrees into 

 the sphere of influence of the earth's own heat, and were there 

 converted into a zone of " watery magma." This intermediate 

 zone between the crust and the firm nucleus is, according to 

 Sterry Hunt, the particular region in which plutonic and 

 volcanic eruptions take origin. ("The Chemistry of the 

 Primeval Earth," Geol. Mag., 1868-69.) 



Dana expressed the opinion that about two-thirds of the 

 earth's mass are composed of iron, and form a rigid nucleus 

 above which a viscous, hot magma forms an intermediate zone, 

 while beyond that zone the earth's crust has a thickness of 

 about seven or eight miles. Amongst other investigators, 

 O. Fisher strongly advocated a molten viscous condition of 

 the earth's nucleus upon which the firm crust rests. Within 

 recent years it has become customary to apply a certain 

 definite terminology to the various zones of the earth's 

 spheroid, in accordance with the supposed physical condition 

 of each particular zonal region. Thus Sir John Murray, in his 

 Presidential Address (Geogr. Sect. Brit. Assoc., 1899), said: 

 " When we regard our globe with the mind's eye, it appears at 

 the present time to be formed of concentric spheres, very like, 

 and still very unlike, the successive coats of an onion. Within 

 is situated the vast nucleus or centrosphere ; surrounding this 

 is what may be called the tektosphere (tektos, molten), a shell 

 of materials in a state bordering on fusion, upon which rests 

 and creeps the lithosphere. Then follow hydrosphere and 

 atmosphere, with the included biosphere (bios, life). To the 

 interaction of these six geospheres, through energy derived 

 from internal and external sources, may be referred all the 

 existing superficial phenomena of the planet." 



Recent seismological observations indicate the transmission 

 of two types of waves through the earth the condensational- 

 rarefactional, and the purely distortional and the study of 

 these tremors supports the view that the centrosphere is 

 not only solid, but possesses great uniformity of structure. 

 The seismological investigations of Professors Milne and 

 Knott point also to a fairly abrupt boundary or transition 

 surface, where the solid nucleus passes into the somewhat 

 plastic magma on which the firm upper crust rests. 



