162 BOAUD OF AGRICULTURE. 



has been the pioneer in our science, for what httle is now known 

 of surface geology we owe chiefly to liis labors. Yet there is 

 an immense amount of work to be done in that direction, and I 

 hope the day is not far when another survey shall be ordered, 

 which will include an investigation of the surface materials 

 which constitute primarily the soil on which agriculture is 

 carried on. 



It is no doubt the case that the rocks are decomposing in 

 places constantly ; and the amount of this decomposition is very 

 great, varying according to climate, and is the result of influ- 

 ences which are also different under different climatic conditions. 

 In our colder northern regions the decomposition is owing 

 chiefly to the filtration of water into rocks, to the frosts follow- 

 ing that infiltration, to the splitting of tbo surface of the rock 

 into fragments in consequence of the expansion of frozen water, 

 and thus the disintegration of the rocks themselves. In more 

 southern climates, where warm tropical rains are incessantly 

 pouring upon the hard rock, it is chiefly that agency and the 

 decaying of the rock by the heat of the tropical sun which pro- 

 duces a similar result. And yet this process, extensive as it is, 

 is not the chief cause, hardly an extensive cause, of the produc- 

 tion of agricultural soil. There is another cause which ought 

 to be taken into account — the wearing of the rock by the action 

 of running water. Here again we have an accumulation of an 

 immense amount of loose materials which are the result of the 

 wearing action of running water. And yet even that is only a 

 small portion of the amount of loose materials which are scat- 

 tered over the earth and form part of the agricultural soil. The 

 main mass of the agricultural soil is derived from an entirely 

 different source, and is produced by an entirely different cause. 

 There has been at work a grinding machine more powerful than 

 the action of the sun, of water, of frost or of wearing cur- 

 rents. It is the agency of ice ; and to that agency we owe 

 not only the grinding of the rocks to powder and all the com- 

 minuted material which forms the chief portion of the loose 

 coatings above the rocks which serve as the basis for our agri- 

 cultural operations, but we owe also to that natural machinery 

 the mixture of rocks derived from different regions, which have 

 formed the compound coating over the whole surface of the 

 earth, without which agriculture would be limited to those 



