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bonate of lime derived from them. These two substances coming 

 together in a state of solution, a chemical reaction, of course, en- 

 sues, both the sulphate of iron and carbonate of lime are decom- 

 posed by the mutual affinities of their ingredients, and the result 

 is a precipitation of the oxide of iron of the former, and a combi- 

 nation of the sulphuric acid and the lime to form sulphate of lime 

 or gypsum. That such is the fact is apparent from our finding, 

 in many cases, a sensible amount of gypsum, either in the earthy 

 state or in minute crystals intermixed with the marl ; and from 

 GUV observing besides, tliat when the gypsum is in greatest plenty, 

 we can most generally discover a strong sulphurous odour, an 

 evidence upon grounds before explained of the existence of sul- 

 phuret of iron, undergoing a conversion into the sulphate of iron. 

 It will suggest itself to everyone, that the existence even in small 

 quantities of so potent a stimulant to vegetation as gypsum, must 

 have a powerful influence in modifying the useful properties of 

 the marl containing it. Yet it must not be inferred from this, that 

 the efficacy of the greensand is owing to the gypsum which I 

 have shown to be frequently present. The comparative inertness 

 of plaster upon the sandy soils of parts of the region where the 

 marl has never been applied, as in several places near Salem, is 

 a fact in itself sufficient to overthrow this notion, even if it were 

 not true that very many marls which do not contain gypsum in 

 any shape, are endowed with the highest fertilizing power. 



Directions for Selecting and .Analyzing the Greensand. 



The valuable fertilizing properties of the greensand, and the 

 extensive use which is made of it in agriculture throughout nearly 

 the whole of the region which it occupies, suggest the propriety 

 of our oflering in this place, some practical directions for select- 

 ing, analyzing, and applying it. 



In seeking for the marl stratum in neighbourhoods where it is 

 likely to occur, but where a covering of any of the superficial 

 deposits obscures it, the primary point to be remembered is, that 

 the true greensand stratum is the lowest accessible deposit of the 

 region. We should find out, therefore, the deepest depressions of 

 the land, such as the meadows and natural ravines, and by the 

 use of an auger or other instrument for boring the ground, 

 several feet in length, ascertain whether the stratum lies suffi- 



