HOW PRESERVED. 117 



indiscriminate aggregation, and individual positions, all in- 

 dicating some sudden death-catastrophe. 



The nature of the sediments must also exercise a marked 

 influence in the number and perfection of the imbedded 

 fossils. Loose and porous sands will be less preservative 

 than impervious clays and muds, and heterogeneous silts 

 than calcareous sediments. In this way minute organisms 

 may totally disappear, and larger ones be preserved in a 

 mutilated and fragmentary form. Every one acquainted 

 with the nature of our peat-mosses, with the sands, clays, 

 and marls that fill up our ancient lakes, and with the sands, 

 gravels, and silts now accumulating in our estuaries and sea- 

 creeks, must have witnessed the different preservative effects 

 of these sediments ; how solid and dense the bones are in 

 one, and how spongy and rotten they appear in another ; 

 how hard and firm the shells are in one, and how soft and 

 friable they occur in another ; and how sharp and clear 

 every external marking is retained in one matrix, and how 

 wasted and obscure it becomes in another. As with these 

 recent accumulations, so with the strata of the older forma- 

 tions; some are destitute of organic remains, which must 

 at one time have been imbedded in them in abundance, 

 while in others they are so imperfectly preserved as to be 

 of little or no value to the palaeontologist. The various 

 ways in which plants and animals may be imbedded and 

 preserved in sediments being so obvious, the different pre- 

 servative effects of different sediments being also apparent, 

 the reason why some forms should occur more abundantly 

 than others being generally discoverable, and the evidences 

 which such plants and animals afford of the geographical 

 conditions under which they flourished being admitted, let 

 us now inquire into the processes by which they are lapidi- 

 fied, or converted into stony matter. 



