CHAPTER III 



EFFECTS OF WEATHER ON THE 

 EARTH'S HISTORY 



THE same causes that produced the layers of peat 

 or sand, or limestone, or clay, which we find by 

 examination of the earth's surface, are acting 

 to-day. Coal is forming now ; and so is limestone ; 

 and so is sandstone ; so even is granite. But these 

 layers or strata form very slowly, so that since man 

 has kept historical records the thickness of new strata 

 laid down could be measured in inches. Consequently 

 we are only able to see the beginnings of the pro- 

 cesses. After the materials were laid down by water 

 or the shifting winds, or by the decay of other materials 

 already in position, they underwent various changes. For 

 example, many layers, instead of consisting of loose mate- 

 rials such as gravel, sand, or mud, are now hard stone. 

 Sometimes this consolidation has been the result of pres- 

 sure. As bed was piled over bed those at the bottom 

 would be more and more compressed by the increasing 

 weight of those laid down upon them ; the water would 

 be squeezed out ; the particles would stick closer to- 

 gether. Mud, for example, might thus turn into clay; 



39 



