268 HIGH SCHOOL ZOOLOGY. 



rations. Further, it is evident, on reflection, that few traces of 

 these preceding generations are preserved, such is the de- 

 structive power of the minute sapi-ophytic organisms that live 

 by pulling to pieces decaying organic matter Even the hardest 

 tissues like bone and wood soon crumble to dust, if left to the 

 oi'dinary process of decay. Now and then, however, the hard 

 parts of a dead animal or plant are preserved, as, for example, 

 when they are washed by a stream to some spot, where they 

 become silted up by the sediment deposited from the stream, 

 and thus protected from the destroying influences to which 

 they would otherwise have been exposed. A reference to the 

 skeletal tissues described for the various groups of animals in 

 the preceding cha))ter, will make it easy to understand what 

 parts would be likel}'^ to be preserved under such circumstances, 

 although under specially favorable circumstances, impressions 

 even of the softest bodied animals are to be found. The material 

 held in suspension by a stream, and deposited as the sedi- 

 ment referred to, is the result of the wearing down or 

 denuding of the land drained by it, and it is obvious that 

 organic remains found in the upper layers of such a deposit 

 must belong to moi'e fecent generations than those found in the 

 lower layers. 



9. Many of the rocks which form the earth's surface have 

 been formed like the silt referred to, from sediment produced 

 by the ceaseless action of the waves on the sea-coast, or by the 

 denuding action of the various atmospheric influences — rain 

 frost, snow, etc., and swept down by fivers to the shallow watei'S 

 of the ocean. Such deposits vary much in their character 

 according to the material held in suspension by the water, and 

 result in sandstones, limestones, shales, etc., but they all 

 exhibit a tendency to layering or stratification, de[)ending on 

 the way they were formed, and are, therefore, called Sedimen- 

 tary Strata. The relative age of such strata is det(a-mined by 

 their position, and it will be readily understood that the upper- 



