90 THE PRINCIPLES OP 



In the next place, there is a class of soils which can scarcely 

 be called conformable to either of these. They have neither 

 been derived from rocks, neither have they been transported 

 from a distance, but, like Topsy in Uncle Toms Cabin, they 

 "growed" where they are found; that is to say, peat 

 soils. These peat soils have a vegetable origin, and they 

 usually rest upon a foundation of solid clay, and frequently 

 of gravel. The history of peat soils has been as follows. 

 There has been an interruption of the drainage of the soil 

 owing in many cases to the overblowing of a natural forest. 

 This interruption of the drainage has caused the growth of 

 a certain class of plants known as peat plants, which have 

 gradually accumulated until they have sometimes reached a 

 depth of a hundred feet. In other cases peat bogs are only 

 a few feet in depth. As just stated, they occasionally attain 

 a depth of one hundred feet, and in such cases we must 

 assume that there has been a subsidence of the surface 

 accompanying an upward growth of the peat. The growth 

 of peat upwards in such cases would in some respects resemble 

 that of Darwin's view as to the growth of a coral reef built 

 up from the slowly subsiding bottom of a tropical ocean. 



Peat soils occur largely in Ireland, and they occur to a 

 considerable extent in this country, in France, and in Ger- 

 many, and, in fact, throughout the northern and middle 

 portions of Europe ; but in the southern latitude of Europe, 

 and lower latitudes generally, the formation of peat ceases. 



Peat soils are rich in organic matter, but they are wanting 

 in mineral constituents. The only mode in which peat soils 

 can be thoroughly developed is by mixing them with the 

 underlying soil, where they make a very useful and even a 

 highly prolific class of land. 



The last class of land remaining to be mentioned is that 

 which is derived from volcanic action. It is known to be a 

 very fertile soil, and it is of interest to us as agriculturists, 



