1 68 HISTORY OF GEOLOGY AND PALAEONTOLOGY. 



In 1834 Henry de la Beche published his admirable work 

 entitled Researches in Theoretical Geology. He described the 

 earth's matter as originally in a gaseous condition, condensation 

 having taken place in consequence of the constant radiation of 

 heat from the earth's surface. Gradually there formed round 

 the inner glowing nucleus a zone composed of heavy metallic 

 substances, beyond which was a region of lighter, molten 

 oxides, and externally a mantle of vapours and gases. The 

 zone, rich in oxygen combinations, afterwards consolidated as 

 a firm crust of crystalline rocks, which protected the inner 

 nucleus and prevented its complete cooling, while the outer 

 vapours condensed in the form of oceans upon the solid crust 



The Cambridge physicist, W. Hopkins, in a series of papers 

 (1839-42) investigated the internal constitution of the earth by 

 means of mathematical calculation. Assuming that the earth 

 was originally molten, then three possibilities are set forth by 

 Hopkins as a result of cooling : 



1. An outer solid crust surrounds a nucleus that is still 



molten, or 



2. The earth's sphere is surrounded by a firm crust, and 



contains a solid nucleus, both separated by a zone 

 of molten material, or 



3. The earth may be completely solid. 



Hopkins calculated that the solid crust of the earth had a 

 thickness of about J or \ of the earth's diameter that is, at 

 least one hundred and seventy-two to two hundred and fifteen 

 geographical miles. A direct communication of the internal 

 molten material with the surface of the earth was therefore 

 impossible in Hopkins's opinion, and he concluded that the 

 volcanoes must draw their molten material from reservoirs of 

 moderate size within the solid crust of the earth. 



At the same time as Hopkins was following out his mathe- 

 matical and physical calculations, Bischof in Bonn was making 

 experiments similar to those which had previously been 

 attempted by Buffon. Bischof caused large balls of basalt to 

 be melted, and observed the time required for the cooling of 

 the melted basalt. By the application of the results to the 

 rate of cooling of the earth, Bischof calculated that the com- 

 plete solidification of the earth would occupy a period of three 

 hundred and fifty million years. Naturally, the application of 

 results obtained upon such a small experimental scale cannot 

 be relied upon in any accurate scientific sense. It was shown 



