380 HISTORICAL GEOLOGY 



the surface of some slabs of rock 250 or 300 fossils may sometimes be 

 counted. 



Animal and plant remains are often well preserved in lake de- 

 posits. In deposits of this class are found leaves, branches, and 

 flowers which were carried from the surrounding land by the streams, 

 insects which were beaten down by the wind to the surface of the 

 lake, and vertebrates which were floated down the streams and 

 found a burial on the lake bottom. Some of the most beautiful 

 fossils were made in this way, but deposits of this class are much less 

 important than those of marine origin, both because of their smaller 

 extent and because the contained fossils seldom afford a means of 

 exact correlation with those of other countries. 



The fossils preserved in delta swamps and flood plains are often 

 numerous, and during certain periods of the earth's history have 

 afforded the chief record of the vertebrates of these periods. 



Fossils are also preserved in wind-blown sand, in peat bogs, in cav- 

 erns, and in travertine. 



Imperfection of the Record. — The record of ancient life must 

 necessarily be imperfect for two reasons. (1) Only a small per- 

 centage of the life of any One period is preserved. This can be seen 

 best by observing the proportion of the plant and animal life of to-day 

 that will remain as a record of the life of the twentieth century. Of 

 the life of the sea only the animals with shells or skeletons will be 

 preserved in large numbers ; the myriads of soft-bodied animals such 

 as jellyfish and protozoans will not form recognizable fossils except 

 under very exceptional conditions. The trees of the forest decay 

 where they fall, and it is seldom that any are buried and leave a per- 

 manent record. The same fate awaits land animals, since upon their 

 death their bones are soon disintegrated by the agents of the atmos- 

 phere and they crumble to dust. It is only the bones of the occasional 

 carcass which floats downstream and is buried under favorable con- 

 ditions that will form fossils. 



(2) Even after being buried, the record is not always preserved. 

 Thousands of square miles of sediments have been metamorphosed 

 and the contained fossils destroyed. When marine sediments have 

 been raised to form land, they are immediately attacked by the 

 weather and erosion and are soon carried away. We consequently 

 find that thousands of feet of rock have been removed and the 

 record has been completely lost. Much of the fossiliferous strata is 

 also either buried so far beneath younger rocks as to be inaccessible 



