4 THE STORY OF THE EARTh AND II \N. 



stones of our present continents, were formed and laid 

 in their places ? With regard to physical conditions, 

 it was a time when our existing continents were yet 

 in the bosom of the waters, when the ocean was almost 

 universal, yet when sediments were being deposited in 

 it as at present, while there were also volcanic foci, 

 vomiting forth molten matter from the earth's hidden 

 interior. Then, as now, the great physical agencies of 

 water and fire were contending with one another for 

 the mastery, doing and undoing, building up and 

 breaking down. But is this all? Has the earth no 

 earlier history ? That it must have had, we may infer 

 from many indications ; but as to the nature of these 

 earlier states, we can learn from conjecture and in- 

 ference merely, and must have recourse to other 

 witnesses than those rocky monuments which are the 

 sure guides of the geologist. 



One fact bearing on these questions which has long 

 excited attention, is the observed increase in tempera- 

 ture in descending into deep mines, and in the water 

 of deep artesian wells — an increase which may be 

 stated in round numbers at one degree of heat of the 

 centigrade thermometer for every 100 feet of depth 

 from the surface. These observations apply of course 

 to a very inconsiderable depth, and we have no 

 certainty that this rate continues for any great dis- 

 tance towards the centre of the earth. If, however, 

 we regard it as indicating the actual law of increase 

 of temperature, it would result that the whole crust 

 of the earth is a mere shell covering a molten mass 



