152 FIELD GEOLOGY. 



was still and quiet ; we conclude, therefore, that it was 

 deposited at the bottom of a deep and quiet sea. 



In the sandstone, on the contrary, the presence of 

 terrestrial remains and the drifted and waterworn con- 

 dition of the other fossils found therein plainly indicate 

 their connection with a shallow sea, the action of cur- 

 rents, and the neighbourhood of land. 



Again, such indications as sun-cracks and the impres- 

 sions of rain-drops tell us of parching heat, of passing 

 showers, and even show us the direction from which 

 the wind blew in those old days of the earth's histoiy. 



We will turn now from such evidence of the sur- 

 rounding conditions at the time when the beds vere 

 formed, and approach the more difficult questions con- 

 nected with the comparative age of the different beds 

 which are found in different localities, and make up the 

 crust of the earth. 



It has been observed, that every group of strata in 

 the geological series, and frequently even the minor 

 beds composing the several formations, have each their 

 own peculiar assemblage of fossils; it often happens that 

 a particular species, or an assemblage of species, are 

 only found in a particular bed or stratum, or if found 

 at all elsewhere it is so rarely in comparison with their 

 abundance in this particular stratum, that they still 

 merit the name of characteristic species. Still more 

 is it the case when a considerable formation or group of 

 beds is examined, for these invariably contain a greater 

 or less number of species which are absolutely peculiar 

 to the group, and are never found in the rocks above or 

 below. 



