ORGANIC AGENCIES. 87 



contiguous country. The central lake, which is seven 

 miles in diameter, is probably the remnant of a once 

 larger lake, as just explained. Fig. 43 is an ideal section 

 illustrating these facts. 



Kate of Growth. In some cases the increase of peat 

 deposits is rapid. In Germany, bogs are known which 

 have formed since the Eoman invasion ; for Roman roads 

 are traced beneath them, and stumps and logs of trees, 

 felled by Roman axes, and even the axes themselves, have 

 been found at the bottom, covered with from ten to fifteen 

 feet of peat. The bogs have been formed by the obstruc- 

 tion of drainage caused by felling the trees. Similarly, 

 many of the bogs of England were formed at the time of 

 the Norman conquest, by the felling of forests, in order 

 to exterminate bands of Saxon outlaws. On the other 

 hand, in the great peat-swamps, where the accumulation 

 is strictly by growth in place, the increase must be very 

 slow, perhaps only a few inches per century. It is evi- 

 dent, then, that the rate is very variable, and therefore 

 no safe estimate of age can be based on thickness. 



Section of a Peat-Bog. A section of a bog reveals 

 the following : 1. Beneath is usually a clay on which are 

 often found the stumps and roots of the preceding forest- 

 growth. . The under-clay seems necessary to hold the 

 water without which peat will not form. 2. Above this 

 a mass of black, carbonaceous matter, structureless to the 

 eye, but showing its vegetable origin to the microscope. 

 3. This passes by gradations through imperfect peat into 

 the recently fallen leaves and branches, and the still grow- 

 ing vegetation. Now, imagine this covered with mud or 

 sand, deeply buried, and subjected to great pressure for 

 ages, and we can easily see that it would become con- 

 verted into a coal-seam, with its under-clay full of roots 

 and stumps, and its roof -shale full of impressions of leaves 

 and flattened stems. 



Alternation of Peat with River-Silt. We have said 



