298 ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. 



resulted from an originally fused globe in the process of cooling, 

 much will have been done toward filling up a great gap in the 

 history of the earth's development. 



I. The Temperature and Density of the Interior of our 

 Pla.net. 



It will no doubt seem to many that the matters to be treated 

 of in this chapter, are far beyond the limits of the subject of 

 the present paper. Since, however, the originally fused condition 

 of our planet, and the constitution of its mass, are at the founda- 

 tion of the igneous view of the origin of the primitive gneiss 

 formation, it would seem necessary to refer to the reasonings upon 

 which the idea of a fused globe, and the various theories pro- 

 pounded regarding the structure of the interior of our planet, 

 are based. Many of these reasonings are founded on phenomena 

 observable at the present day, which point to the existence of 

 intense heat and extraordinary density in the centre of the earth. 

 Hence this proposed recapitulation of the evidences of internal 

 'heat and density may net be out of place. 



Whatever may have been the temperature of the earth's 

 surface in the former periods, it is abundantly evident that it is 

 now altogether regulated by the sun. Since the influence of 

 the sun's rays penetrates to some extent beneath the surface, and 

 affects the degree of temperature there existing, it will be neces- 

 sary to define the extent to which" this takes place, before pro- 

 ceeding to advert to the influence of the subterranean heat on 

 the temperature of the earth's crust. It is obvious that the 

 influence of the sun's rays is exerted very irregularly, and that 

 •variations in the degree to which the surface of the earth is 

 affected by it occur throughout the day, and annually. The 

 diurnal variations are of course not so great as those of the year, 

 and the latter vary of course with the situation of the point of 

 observation. These diurnal and annual variations are less and 

 less felt, the deeper, to a certain point, we penetrate beneath the 

 surface. Towards this point the extremes of temperature grad- 

 ually approach nearer to each other, the differences are gradually 

 equalized, and finally they disappear completely. The depth 

 at which this point of constant temperature exists varies with 

 latitude and climate, and with the capacity for conducting heat 

 which the surface possesses. In African deserts, where the sand 

 has been found to possess sometimes a temperature of 40° to 48° 



