SOIL. 



ements. The science of geology, 

 which teaches the relative ])osilion 

 and nature of the minerals ol' which 

 the outer crust of the earth is formed, 

 is consequently of the greatest util- 

 ity in aiding us to compare different 

 soils, and ascertaining their compo- 

 sition. 



" The knowledge which geology 

 imparts is, however, not sufficient 

 for the minuter classification of soils ; 

 for it is found by experience that the 

 soils which lie over or near the dif- 

 ferent strata, as they appear near the 

 surface, vary greatly, although they 

 retain some general character which 

 distinguishes them from others. The 

 streams which descend from the hills, 

 and flow towards the valleys, and 

 through them to the sea, carry to a 

 great distance the minuter portions 

 of the minerals which they flow over 

 in their course, while the larger and 

 heavier are deposited much sooner. 

 Hence the heterogeneous mixture of 

 various earths and stones, and their 

 stratification in thin layers, as is oft- 

 en found when a soil is examined 

 which has never been disturbed by 

 cultivation. A sudden tiood, rising 

 rapidly, carries stones and fragments 

 of rocks in its course, while a gentler 

 stream deposites fine sand or clay 

 over these, and forms every variety 

 of sandy, gravelly, or clayey soil. 

 If chalky hills are near, carbonate of 

 lime abounds in almost every propor- 

 tion, with its usual concomitant, ir- 

 regular flints. If the waters have 

 accumulated in a basin, and formed 

 a temporary lake, the soil will con- 

 sist of all the finest portions of the 

 minerals, which, from their minute 

 size, have remained long suspended 

 in the still waters, and slowly depos- 

 ited in the form of mud. In propor- 

 tion to the shallowness of the lake, 

 vegetable matter will have been pro- 

 duced, and intimately mixed with the 

 minerals ; and, where vegetation has 

 gone on rapidly, peat and soft bogs 

 are formed. 



" It is not sufficient to class soils 

 according to the substance which pre- 

 dominates, as has been usually done, 

 such as sandy, gravelly, chalky, or 



728 



clay soils ; for this gives very imper- 

 fect information respecting their na- 

 ture or fertility ; neither is it alto- 

 gether sufficient to class them ac- 

 cording to any particular geological 

 formation. It is important to enter 

 into a more minute examination of 

 their component parts. But as the 

 geological investigation of the differ- 

 ent strata is a great help in the ex- 

 amination of soils, we will in the first 

 place give a short description of those 

 which have the most distinct charac- 

 ters, from their connexion with dif- 

 ferent geological formations. 



" The soils which are immediately 

 derived from those rocks in which no 

 traces of organic remains are to be 

 found consist either of visible frag- 

 ments of quartz and other hard min- 

 erals, which are not affected by ex- 

 posure to air or water, and are only 

 ground and comminuted by being rub- 

 bed against each other in floods and 

 torrents, or of minuter particles of 

 the same, of which the shape is not 

 readily distinguished by the naked 

 eye. When they are altogether com- 

 posed of visible particles and stones, 

 the water readily passes through 

 them ; and unless they are kept con- 

 tinually moist by a regular irrigation, 

 without any stagnation of the water, 

 they are absolutely incapable of sus- 

 taining vegetation, or of bringing 

 fruits to maturity. It is seldom, 

 however, that any gravel or sand 

 does not contain some portion of 

 earth or other matter, of which the 

 particles become invisible when dif- 

 fused through water, and to which, 

 for the sake of perspicuity, and to 

 prevent confusion, we will here give 

 the general name of impalpable sub- 

 stance. A certain portion of this finer 

 part of the soil, and its due admix- 

 ture with the coarser, especially 

 where there is some regular grada- 

 tion in size, and no stones of too large 

 dimensions to obstruct the instru- 

 ments of tillage, may be considered 

 as essential to fertility. The chem- 

 ical composition of the impalpable 

 substance, no doubt, greatly affects 

 the degree of fertility ; but the gen- 

 eral texture must be considered as 



