406 Metamorphism. 



the relation that these changes seem to bear to laws that we 

 can study, and forces that we can measure. 



The work that goes on near the earth's surface takes its 

 origin at the contact of earth, air, and water, and the action of 

 water and weather upon all exposed rocks is constantly tending 

 to grind and pare away inequalities. Thus there is a never- 

 ceasing piling up of mud, and stones, and sand, in the great 

 expanse of the ocean, but especially near the shores, where the 

 waves are always beating, and towards which the rivers bear 

 their tribute. Marine currents spread this over the sea bottom, 

 and with it is mixed a vast, but very variable quantity of 

 organic matter, either bones, teeth, shells, corals, or the micro- 

 scopic cases of animalcules, according to circumstances and 

 conditions which we can occasionally study, but with which we 

 are not perfectly familiar. The rate of such accumulations over 

 the whole earth must certainly be very large every year, and if 

 there were no counteracting cause, the result would in time 

 reduce the land to the merest shreds and patches of the hardest 

 rock. 



But that there is some counteracting cause is certain, for 

 its results are traceable without much difficulty. In many 

 parts of the earth there are movements of elevation or depres- 

 sion — limited, no doubt, at present, to definite areas, and, per- 

 haps, not unconnected with those districts, where earthquakes 

 are felt, and where volcanoes cast out from yawning cracks in 

 the earth volumes of steam, fiery floods of lava, and thick 

 clouds of ashes. Very slow are these movements, but, perhaps, 

 not much slower than corresponds with the amount of accumu- 

 lations at the sea bottom, but the result is that all the great 

 heaps of mud, and sand, and pebbles, and angular stones, mixed 

 up with the remains of animals and vegetables, become buried 

 — carried down bodily away from the surface, and covered up 

 with similar deposits. 



Material thus depressed and squeezed, after beiug deposited 

 in water, becomes exceedingly compact and tenacious, as is 

 known by the state of the mud at the bottom of the Atlantic. 

 Under such circumstances, it parts with some of the water with 

 which it was formed, and it tends to become solid. It then by 

 degrees eaters into a region where the temperature is equable. 

 As it goes deeper it is exposed to greater heat, and it is highly 

 probable that the law of increase of temperature in descending 

 into the earth, which is pretty uniform for 2000 feet, continues 

 nearly the same to much greater depths. By degrees, then, the 

 descending mass becomes heated, and at the same time it is 

 constantly under pressure. 



That the currents of electricity producing the phenomena 

 of earth-magnetism chiefly act within a certain distance from 



