320 GEOLOGY. 



time. There is nothing in science which so well im- 

 presses the mind with the truth of the declaration of the 

 Bible, that " one day is with the Lord as a thousand 

 years, and a thousand years as one day." 



433. Minute Agencies. I have brought to your notice 

 in this book many examples of extensive operations in 

 the construction of the earth by minute agencies. It is 

 a very small thing for a coral animal to separate from 

 the water a little carbonate of lime and appropriate it to 

 itself; but multitudes of these animals, at work in the 

 same locality, year after year, for centuries and even 

 ages, lay down thick masses of limestone rock. The case 

 is even stronger with the diatoms, microscopic vegetable 

 organisms which separate silex from its solution in the 

 water and deposit it in thick beds. A large portion of 

 the crust of the earth is, in fact, the result of the aggre- 

 gate labor of minute animals and vegetables. 



434. Disintegration of the Rocks. In the preparation 

 of the earth for the use of man, the disintegration of the 

 rocks has been a prominent process. The soil which he 

 cultivates is, as you have seen, a result of this disintegra- 

 tion. The mud that is carried down the rivers, and de- 

 posited in deltas or on flood-plains, is comminuted rock 

 gathered by the waters from mountains and hills. The 

 ice, in glaciers and icebergs, is continually grinding up 

 the rocks to add to the soil of the earth. A vast work 

 has been done in the past by water, in both its liquid 

 and solid forms, in this preparation of the earth for veg- 

 etation. This was done largely, as you have seen, in the 

 Glacial period ; but the work has always been going on, 

 for water has always been in motion. 



435. Reconstruction. In the building up of the earth 

 there has been a vast amount of reconstruction. The 

 rocks of which the crust of the earth is composed are 

 made, to a very great extent, of materials derived from 

 the ruins of rocks previously made ; and often the ma- 

 terials have been used over and over again. Disintegra- 



