HISTORICAL GEOLOGY 



379 



(3) Casts and Molds. — The original substance may be carried away 

 in solution by underground water, leaving a cavity in which only 

 the external form is preserved ; in other words, a mold of the shell or 

 bone is left. Often natural casts of these molds are formed by mineral 

 matter carried into the mold or by the infiltration of mud. Molds 

 of the interior (Fig. 362) and exterior (Fig. 363) are frequently en- 

 countered in porous rocks. Some fine opals in Nevada have the form 



Fig. 362. — Specimens showing the 

 original shell (B) and a natural mold 

 (A) of the interior of a similar speci- 

 men from which the shell has disap- 

 peared. {Turritella mortoni.) 



Fig. 363. — One half of a con- 

 cretion showing the leaf which 

 formed the nucleus. 



of branches, but are in reality casts of the branches of trees, the 

 cavities formed by the decay of the wood having been filled with 

 silica. 



(4) Footprints, Trails, etc. — Many animals are known from their 

 footprints, trails, burrows, or the impressions (Fig. 380, p. 411) made 

 by their bodies in the soft mud. 



Entombment of Plants and Animals. — The most favorable con- 

 ditions for the preservation of animal life are to be found on those 

 portions of the ocean bottom which are not uncovered by tides and 

 where sediments are accumulating. When under such conditions 

 shellfish or other animals die, their bodies may be buried in the mud 

 or sand and preserved. It is not unusual to find layers of rock made 

 up largely of the remains of shells which were buried in this way. On 



