698 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



On the Chamberlin hypothesis the isothermal surfaces within the earth are 

 roughly parallel to its own surface because of the progress of the central heat- 

 wave along all radii, but the temperature of molten lava at atmospheric pressure 

 (say, 1100° C), is to be found, on the average, at a depth of about 400 miles. 

 The difficulty of understanding how magma can pass through so thick a shell 

 is not removed by Chamberlin's suggestion that liquid ' tongues ' of the more 

 fusible rocks are forced through the shell by the tidal kneading of the earth. 

 The differences in the so-called ' fusibility ' of the principal igneous rocks are 

 really not great, the ' fusion points ' for dry melts generally lying between 1000° 

 O. and 1150° C. When water and other volatile fluxes enter the melts the 

 differences may be still less. As a matter of fact the acid, more difficultly melted 

 rhyolites are often directly associated with the easily melted basalt in the same 

 region and in the same petrogenic cycle. If differential ' fusibility ' controls 

 the eruption of magma, the material of great depth must be something differ- 

 ent from the known mixtures of silicates in nature. 



The geologist is mainly interested in the planetesimal hypothesis as it 

 bears on the condition of the earth since the time when it approached its present 

 size through accretion. When the planet has reached that size it is entirely 

 possible that the central heat has increased beyond the critical temperatures of 

 all known substances. This would in turn imply complete change in the dyna- 

 mics of the earth. The change of state in the core implies: (1) enormous gaseous 

 pressures; (2) the differentiation of the original miscellaneous materials into 

 a system of fluids which would be stable under the new conditions; with (3) an 

 inevitable transfer of hotter, because originally more centrally placed, material 

 to higher levels; (4) the consequent melting of the solid matter overlying this 

 new gaseous system; (5) the expected evolution of the more volatile matter at 

 temperatures high enough to further the melting of the solid overlying shell. In 

 fact, there is in Chamberlin's hypothesis no known reason why the whole planet 

 from surface to center should not become fluid in this relatively late stage in 

 its development. For the period including what is generally called geological 

 time there would, in this view, be no essential difference between the gas-nebula 

 and planetesimal-nebula hypotheses. 



In any case the analogy of the sun and other stars, the yet closer analogy 

 of the ' semi-sun,' Jupiter, and, indeed, the face of the moon, all suggest that 

 the earth was once wholly fluid in its surface shell at least. The plain evidences 

 of such present or former superficial fluidity in other members of the solar 

 system are not sufficiently regarded by any hypothesis which denies a similar 

 stage in the earth's history. 



According to either of the two rival cosmogonies now holding attention, 

 magmatic heat may be chiefly explained as an inheritance from a primitive 

 condition of the earth when it was fluid at the surface. In either case a crust 

 was formed through loss of heat by radiation, with a magmatic stratum beneath. 

 The calculations of Kelvin and others show that the temperature of any volcano 

 is to be found at a relatively shallow average depth within the earth. Extra- 

 polation on the normal temperature gradient (3° C. rise per 100 metres of depth) 



