126 EVOLUTION AND GENETICS 



sediments or wind-borne material to afford it an effective degree 

 of preservation. Once buried the original tissues of the organism 

 may be preserved, their form may be reproduced by the addition 

 or substitution of minerals (petrifaction), or the mold of the 

 body in the material which surrounded it may persist after the 

 tissues have disintegrated. Molds are sometimes filled in with 



Fig. 67. — Bercsuvka mammoth, Eicpnas pritiujciuus, discovered frozen in 

 the soil. Specimen as it now appears in Petrograd. (From Lull.) 



other mineral matter, forming casts of the original (Fig. 66). In 

 view of these processes, it is only natural that the sedimentary 

 rocks contain the majority of the known fossils, since they are 

 formed from materials which are the most likely to preserve 

 organic remains. 



The perfection of fossils of the several kinds varies greatly. 

 Records are available of the preservation of animals in Siberia by 

 a natural cold storage in ice or soil. The remains of a mammoth 

 (Fig. 67) preserved in this way were found in 1901 at Beresovka, 

 Siberia, 60 miles north of the Arctic Circle. ''This creature evi- 

 dently slipped into a natural pitfall of some sort, possibly an ice 

 crevasse covered with soil and vegetation. A fractured hip and 

 fore limb, a great mass of clotted blood in the chest, and un- 



