196 



Besides the aluminous and siliceous matters here recorded as 

 usually present with the greensand in the general mass, there 

 occur occasionally several other substances, which, though com- 

 paratively minute in quantity, are possessed of active properties. 

 Some of these materials are probably deleterious, while some are 

 undoubtedly beneficial in their action upon vegetation. The 

 substances referred to are carbonate of lime, already mentioned, 

 sulphate of iron, sulphate of alumina, sulphate of lime, and sulphate 

 of magnesia ; also, phosphate of iron. 



They appear to be derived, mainly at least, from constituents 

 in the clay, and only very partially, if at all, from elements in the 

 greensand itself. 



The carbonate of lime, in most instances, we can trace to fossil 

 shells and other organic remains, imbedded in the stratum. The 

 sulphate of iron, obviously proceeds from the action of the at- 

 mosphere and moisture on the sulphuret of iron, so abundant in 

 the clay ; and the sulphate of alumina from the union of a portion 

 of the sul huric acid thus developed, with the argillaceous earth 

 of the clay; while the sulphates of lime and magnesia may result, 

 either from the combination of the same acid with some of the 

 lime and magnesia, sometimes present in a minute share in the 

 green mineral, or more probably from its reaction on the car- 

 bonates of lime and magnesia, existing like the sulphuret of iron 

 in an insulated state. The phosphate of iron, is no doubt derived 

 from phosphoric acid, traceable to the animal remains, acting on 

 oxide of iron. 



Several of these substances develope themselves upon the mass 

 of the marl after it has been dug and exposed to the atmosphere, 

 in the form of a white efflorescence, encrusting alike the clayey 

 matter and the granules of greensand, with a delicate crystalliza- 

 tion resembling a light frost. Collected and carefully examined 

 and analyzed, this efflorescence will be found almost invariably 

 to consist when it is of a pure white, of either the sulphate of 

 magnesia, or sulphate of lime, (gypsum,) the latter predominating ; 

 and sometimes, these two occur united. In some instances, we 

 recognise it to contain the sulphate of magnesia, (epsom salts,) in 

 sufficient quantity to be distinguishable by its taste. A yellowish 

 tint and an astringent flavour are apparent when it consists chietly 

 of the sulphates of alumina and iron. The carbonate of lime 



