68 ILLINOIS ACADEMY OF SCIENCE. 



the surface by extrusive action which would tend to increase the 

 mean specific gravity of the residue. The curve of Dr. Lunn may 

 be regarded as a second approximation.^^ But this, as noted, does 

 not take into consideration the effects of Hquefaction and extru- 

 sion and these, in the planetesimal view, are of the first order of 

 importance. The theoretical curve mathematically deduced by 

 Dr. Lunn is, however, an indispensable basis for a third approxi- 

 mation in which the effects of liquefaction and extrusion are 

 taken into account. 



Before passing on to consider Hquefaction and extrusion, it is 

 well to note that the Lunn curve based on the Laplacian law of 

 density also is low near the surface and that its rate of rise is 

 much below that of the temperature gradient observed in wells and 

 mines. Dr. Lunn, on assumptions carefully specified in his dis- 

 cussion in the paper cited, found the rise in the first 200 miles 

 only 330° C. 



This low development of heat in the outer part of the earth 

 seemed at first thought to present a difficulty of a rather serious 

 nature, but it was believed to be met by the effects of liquefaction 

 and extrusion, and these were made the chief basis of an addi- 

 tional approximation to the actual temperature curve (Vol. I, 

 Chamberlin and Salisbury's Geology, pp. 265-67). It was held 

 that the rising heat of the interior would reach the temperatures 

 of fusion or of mutual solution of some ingredients in the mixed 

 material much earlier than that of other ingredients, and that the 

 ascent of the portion that became molten carrying its latent as 

 well as sensible heat into the cooler outer zone would necessarily 

 raise the temperature of that zone. It was held that the continua- 

 tion of this process served as a constant influence tending to retard 

 the rise of temperature in the deeper zone where the partial 

 liquefaction was in progress while it progressively raised that of 

 the outer zone into which the liquid rock was intruded, whether it 

 lodged in the crust or passed through it to the surface. This 

 extrusive process was supposed to have continued to the present 

 day and to have resulted in a permanent adjustable working curve 

 of accommodation between thermal, fluidal and mechanical condi- 

 tions. This curve, except in the cool crust, was essentially identi- 

 cal with the fusion-solution curve, whatever that might happen 



^Year Book No. 3, Carnegie Institution of Washington, 1904, p. 156; also Geo- 



fhysical Theory Under the Planetesimal Hypothesis, Section II of Tidal and Other 

 'roblems, Publication No. 107, Carnegie Institution of Washington, 1909, pp. 169- 

 231; for a summary and figure of curve see also Geology, Chamberlin and Salisbury, 

 Vol. I, p. 566, 1904. 



