FOSSILS AND PEDIGREES 



343 



book which has been extensively 

 damaged by fire, water, and 

 decay, so that many of its pages 

 are ahogether missing, while 

 others are variously torn, dog- 

 eared, crumpled, and their con- 

 tents rendered illegible. The 

 following passage emphasising 

 the imperfection of the geological 

 record was written in 1898 by 

 Sir Arthur Smith Woodward, 

 one of the leading authorities on 

 fossil fishes, but in spite of many 

 discoveries made since that date, 

 its substance remains funda- 

 mentally true to-day. "We may, 

 in fact, without exaggeration 

 declare that every item of know- 

 ledge we possess concerning 

 extinct plants and animals de- 

 pends upon a chapter of accidents. 

 Firstly, the organism must find 

 its way into water where sediment 

 is being deposited and there 

 escape all the dangers of being 

 eaten; or it must be accidentally 

 entombed in blown sand or a 

 volcanic accumulation on land. 

 Secondly, this sediment, if it 

 eventually happens to enter into 

 the composition of a land area, 

 must escape the all - prevalent 

 denudation (or destruction or 

 removal by atmospheric or 

 aqueous agencies) continually in 

 progress. Thirdly, the skeleton 

 of the buried organism must 

 resist the solvent action of any 

 waters which may percolate 

 through the rock. Lastly, man 

 must accidentally excavate at 

 the precise spot where entomb- 

 ment took place, and someone 



TABLE or STRATA 



ERAS 



PERIODS 



DOMINANT 

 TYPE5 



REC ENT 8t PLEISTOCENE- 



PI.IOCENC •';: 



OLIGOCENE 



CRETACEOUS 



JURASSIC 



TRIA3SIC 



O ^I 



DEVONIAN 



SILURIAN 



CAMBRIAN 



ARCH/€:an 



i 



CARBONirEROUS 



■m. 



(n < 

 < 



z 



eO 



Fig. 122. — TABLE OF BRITISH STRATA, 

 SHOWING APPROXIMATE THICKNESS. 



