STRUCTURE OF THE VERTEBRATES 331 



were cut and metamorphosed by igneous intrusions before the 

 accumulation of an atomosphere; but as soon as winds and rains 

 appeared these original rocks were decomposed and washed or 

 blown into the valleys. Synchronously the accumulated moisture 

 would fill the depressions and form lakes and seas, which would 

 be gradually filled by the sediment of sand and mud. It is in 

 these deposits that the remains of plants and animals are caught 

 and buried. Rocks are formed from the sedimentary layers as a 

 result of pressure and cementation. Layers of sand become 

 sandstone, mud becomes shale, and deposited calcium carbonates 

 are turned into limestone. The last are particularly interesting, 

 for it is generally conceded that limestone deposits show the 

 presence of living organisms. The carbon dioxide of metabolism 

 unites with the calcium oxide nearly always found in water, and 

 calcium carbonate is precipitated. Therefore, the fact that highly 

 altered limestones are among the earliest known rocks, indicates 

 the presence of some form of life much older than any rec- 

 ognizable fossils. 



Sedimentary rocks are usually laid down in strata. And 

 change in composition of the sediment would cause a distinct 

 line of demarcation. Even so slight a change as the fallen 

 leaves of autumn would distinguish those strata from the clearer 

 ones of spring. When the strata are undisturbed the older ones 

 are at the bottom, and this condition is found in many places 

 of the earth. But fortunately for the palaeontologist, the move- 

 ments of the earth's surface have tilted the strata, or turned 

 them on end, so that miles of strata can be studied which would 

 never be found if each stratum had remained in place. The earth 

 movements are so vast that the relative position is rarely dis- 

 turbed, and the sequence of history can be traced. A more dis- 

 turbing problem for the palaeontologist is presented when a 

 region shows that the ocean bottom has risen above the surface, 

 been eroded, and then sunk again with more recent strata above. 

 In such cases the line of erosion (unconformity) is usually dis- 

 tinct, and the gap can be partially filled from a knowledge of 

 other undisturbed strata. 



By a study of the rocks a surprisingly clear record of the 

 earth's story has been deciphered. After the fundamental posi- 

 tion of the strata had been determined and the fossil life care- 



