6 AN INTRODUCTION TO THE STUDY OF FOSSILS 



process due largely to the multiplication of bacteria. Hence 

 in these more landward deposits an organism encounters many 

 agencies of destruction, both chemical and organic, and stands 

 little chance of preservation (Fig. i). Thus in the Mauch 

 Chunk shales of Pennsylvania and in the Newark beds (an 

 Upper Triassic alluvial fan and flood plain deposit) of Connecti- 

 cut, New Jersey, etc., few fossils occur and these are principally 

 footprints and plant impressions. In portions of the Wasatch 

 and Bridger formations (Eocene flood plain deposits) of Utah and 

 Wyoming more fossils occur, but they are much more abundant 

 in the Florissant beds (a Miocene lake deposit) of Colorado. 



In short, the more completely air and circulating water are 

 kept away from organic remains the better the chance for their 

 preservation as fossils ; and, accordingly, marine, lake or marsh 

 deposits are more favorable than those of a flood plain, alluvial- 

 fan or playa, while clay or limestone are better mediums than 

 sandstones or coarse ash deposits. 



It is thus seen that the chances for the preservation of animals 

 or plants varies with their habitat (Fig. i). Inhabitants of 

 mountainous regions where erosion is greater than deposition stand 

 very poor chance for preservation in the fossil state ; those living 

 on low plains and in fresh water are more likely to be preserved ; 

 but it is the marine forms, especially those living upon the sea- 

 floor beyond the breakers but in water still comparatively shal- 

 low, that stand the best chance for preservation (Fig. i). Here 

 at their death the organic forms are quickly covered by the sedi- 

 ment which, brought in by the rivers or torn by the waves from 

 the land, is widely distributed by the drift of the waters. Hence 

 of all organic forms, marine invertebrates stand the best chance 

 of preservation, for they are the most numerous of all animals and 

 they occupy those areas w^here oxidation is slow and where a 

 prolonged sedimentation is apt to occur ; these forms are, 

 accordingly, the most important of all fossils for the cor- 

 relatiofi of strata. It is not strange, therefore, that many 

 species of fossil insects are known each from but a single speci- 



