G. H. Kinahan — On the Growth of Soil. 267 



usual sub-soil, the limy Boulder-clay drift they never burrow in, 

 and a sub-soil frequent in some places, namely, a stratum of bog-iron- 

 ore, they could not possibly enter ; yet over all these different kinds 

 of sub-soil the surface mould increases if the land is laid down in 

 permanent grass. The beginning of the soil forming the land over a 

 subsoil of bog-iron-ore, cannot possibly be due to anything but 

 chemical action and vegetable decay, as no worm-work could have 

 been done ; and if in such an instance a surface soil can be foimed 

 without their aid, why is it that similar work does not go on in 

 other places ? Moreover, in such places the work is carried on under 

 most adverse circumstances, for eveiy one ought to know that a bare 

 surface of limonite is not a very favourable place for vegetation, and 

 usually it appears due to the decay of the water lodged on the 

 surface, with perhaps a slight disintegration of the underlying 

 mineral, from which are generated lichens and mosses, and eventually 

 a peaty soil. It, however, is added to very gradually, for in dry 

 weather, there being no depth of earth, the vegetation is withered 

 away. 



Land that for years has been tilled to one depth will have a 

 surface to its sub-soil like a road, and this is so well known, that 

 to counteract it sub-soiling has been introduced. If such land is laid 

 down in grass and subsequently again broken up, the surface of the 

 sub-soil, caused by the former tilling, will be found intact ; but the 

 depth of the mould will have increased in proportion to the number 

 of years it has remained under grass. This increase cannot take place 

 below, as the old sub-soil surface still remains, therefore the mould 

 must have increased in thickness upwards, and necessarily by the decay 

 of the vegetable substances. It may be said that this is an excep- 

 tional case ; for naturally this road-like surface would not be formed 

 over the subsoil. However, if the thickness of the mould increases 

 downwards, is it not remarkable that over each different kind of 

 sub-soil the thickness of the vegetable mould should be so uniform ? 

 Naturally the mould is only a few inches thick over gravel, a little 

 thicker over clay, a good depth over a sub-soil formed of a combi- 

 nation of clay and sand, more especially if it is limey, while over an 

 alluvial sub-soil it may be any depth ; unless, indeed, that chemico- 

 fluvial denudation in each several case has removed an exact equiva- 

 lent from the surface for the increase below ; but this is highly 

 improbable. 



Before leaving the subject it should be mentioned that there are 

 remarkable facts in connection with the decay of drift and its change 

 into silt, that may bear on the growth of soil. One of these is well 

 exemplified in a river that is partly subterraneous and partly sub- 

 aerial ; flowing partly between banks of Boulder-clay-drift and 

 disappearing, not in an open vent, but through a filter formed of 

 the insoluable parts of the drift. Such a stream, during floods, 

 wears and cuts away the drift banks, and when it dries up there is a 

 thick coat of fine silt over the natural filter ; while in the bed of the 

 stream there will not be blocks and fragments of rock in due pro- 

 portion to those that were in the di'ift denuded away during the 



