6 THE STRUCTURE OF THE EARTH. 



action of matter under pressure and heat, there is far more reason for 

 believing the eartli to be liquid than for taking the opposite viev/. 



From what has here been stated it would seem that there is no evidence 

 drawn from mathematical and physical laws which obliges the petrographer 

 and geologist to assume an interior structure for the earth different from 

 that which the facts of geology and petrograph}' would lead them to expect.* 



Starting, then, with the accepted belief that this earth was once an 

 intensely hot gaseous body, it follows that if the heavier gases tend to 

 lie nearer the centre than the lighter ones, the dissipation of heat could 

 only take place through the slow conductivity of gases. In like man- 

 ner, when the earth cooled down to a liquid mass convection would soon 

 cease, if it ever existed, on account of the different densities of the earth's 

 materials; and here also the dissipation of heat would have to tnke place by 

 the slow conduction of liquids. In the same way, in the solid portions of the 

 earth the heat from the interior has to be conveyed outwards through broken, 

 fissured, heterogeneous material. It would seem that all these conditions 

 should be taken into account in all physical discussions of the age of the 

 earth and sun ; but thus far all calculations seem to have been based upon 

 the laws of the relation of gases and liquids of about the same density. 

 There should further be considered the heat disengaged by the chemical 

 unions necessary to form the jjresent mineral combinations now existent on 

 the earth. 



As the liquid earth cooled and its materials grew viscous, all interchange 

 of materials would be retarded; and as the cooling continued, the lighter 

 exterior liquid portion would form a hot crust, which would be lighter than 

 the underlying liquid. On account of the viscous condition through which 

 the earth's materials must pass befoi-e solidification, the. crust would gradu- 

 ally shade into the underlying liquid, and both would be nearly in the same 

 condition with each other as to temperature. It is not probable that the 

 crust would break up and begin to sink, because, even if its surface grew 

 cold, it woidd always have this hot solid base lighter than the underlying 

 viscous liquid, which, owing to the increase of specific gravity as the in- 

 terior is approached, would probably be more dense than any of the over- 

 l^ing cold crust. Even if the crust should become heavier, break up, and 

 begin to sink, this sinking woidd be very slow, on account of the viscosity 



* Whitney, Earthquakes, Volcanoes, and Mountain Buihling, 1S71, p. 7i ; Dana, Man. Geo]., 18S0, 

 p. 812. 



