20 BULLETIN OF THE NATURAL HISTORY SOCIETY. 



formed at a red or almost white heat. As it cooled 

 down, these minerals become unstable, that is, they tend 

 to change chemically into other combinations, and this 

 change is greatly assisted by exposure to air and rain. 

 Part of their substance is dissolved away, the rest left as 

 loose grains, or changed to new minerals, which are 

 usually softer than the original ones, and are not in large 

 and solid crystals, but in a fine, loose powder, or mud, 

 so that they are easily washed away by the water and 

 carried down by streams to the sea, where they are 

 deposited as sediment. This chemical change, or weather- 

 ing, is one to which all rocks are subject, and it is the 

 great agent by which they are worn away, the grinding 

 and battering action along sea-coasts playing but a sub- 

 ordinate part. 



If the sediments are deposited in a place where the 

 eartb's crust is slowly sinking, more and more material 

 is piled up on top of them, weighing them down with 

 an enormous pressure, and they gradually settle into a 

 campact mass. The water contained in this mud is under 

 a very heavy pressure as well, and as the mass sinks 

 deeper it comes more and more within the influence of 

 the earth's internal heat. By this com.bination of heat 

 and pressure the solvent power of water becomes much 

 greater, and it dissolves out a considerable part of the 

 rock. As the mud settles downward and becomes more 

 compact, it is evident that the water will work its way 

 upward, so that throughout the mass there will be 

 relativel}^ a continual, slow, upward seeping of water, 

 charged with all the dissolved rock it can carry. This 

 water will come into zones of less heat and pressure as it 

 rises, and must then deposit some of its mineral matter, 

 cementing the compacted sediment into a solid rock. 



