132 LIFE ON THE EARTH. 



species over the area, and that during this period 

 one foot of shale was accumulated, we have the 

 years consumed in depositing the strata of this coal- 

 field, 3000 to 5000ft. thick; =15,000 to 25,000 years. 

 But several layers of the shells lie in the thickness of 

 one foot. In the same coal-field, near the base, we find 

 one or more thin beds of shale and calcareous balls, 

 abounding in Goniatites Listen, and some other ma- 

 rine shells. These Goniatites are of every magnitude, 

 from the youngest not larger than mustard-seed, to 

 specimens three inches in diameter. This fact leads 

 to an inference of the same order, and indicates a 

 period only to be expressed in tens of thousands of 

 years. 



These inferences apply only to the rate of accu- 

 mulation of shales ; we cannot adopt the same esti- 

 mates for the sandstones, which may have been 

 more rapidly, or for the coal-beds which may have 

 been more slowly aggregated. For these latter de- 

 posits, fortunately, Liebig and modern agricultural 

 Chemistry have supplied a different and independent 

 basis of computation. 



It is now very generally admitted that our coal- 

 strata are derived from the accumulation of trees, and 

 other kinds of plants, on or very near to the places 

 of their growth ; at all events the greater part can have 

 been moved but small distances. Plants after their 



