88 DYNAMICAL GEOLOGY. 



that peat occurs in the river-swamps and deltas of great 

 rivers. It is easy to see, therefore, how peat deposits 

 may at long intervals be flooded and covered with river- 

 silt, and again reclaimed and covered again with peat 

 vegetation, perhaps many times. Now, in cutting into 

 the delta of the Mississippi, several layers of peat, with 

 interstratified silts, are found. The resemblance of this 

 to a series of coal-seams on a small scale is very striking. 

 It is by observing things now going on that we find the 

 key for interpreting things which occurred in earlier geo- 

 logical times. We shall apply these principles in Part III. 



Drift-Timber. 



But there is another way in which vegetable accumula- 

 tions occur now, and therefore may have occurred in 

 previous epochs. Great rivers in heavily wooded coun- 

 tries, like the Mackenzie and the Mississippi, in flood- 

 times, bring down large quantities of drift-timber gath- 

 ered in their upper courses, and accumulate them in the 

 form of rafts at their mouths. These natural rafts are 

 often of great extent. One, at the entrance of the Atcha- 

 falaya, near the head of the delta of the Mississippi, was 

 in 1838 ten miles long, a quarter of a mile wide, and 

 many feet thick. Such rafts become finally water-logged, 

 sink, and are covered up in a river-silt. Then they are 

 slowly changed into a brownish, cheesy substance, and 

 doubtless finally into lignite or coal. Now, in cutting 

 into the delta deposit of the Mississippi, layers of drift- 

 timber are met with which is undergoing this change. 

 This also may throw light on the formation of coal and 

 lignites. 



SECTION II. IRON ACCUMULATIONS. 



Every one must have observed that in (wrtsim boggy 

 spots, on hillsides, or on plains at the foot of hills, are 

 found reddish deposits of iron mixed with earth. This 



