192 CHEMISTRY OF THE EARTH. 



tusecl and oxidized state in recent sediments, must lie less in those of 

 more remote times.* 



To tliG chemist the presence of graphite, or of a metallic sulphide in 

 a rock, aflbrds clear evidences of the intervention of organic life ; and 

 these indirect evidences are met with not only in the oldest known strati- 

 fied rocks, those of the Laurentian system, but in the eruptive 

 diorites, which rise from beneath them, and are pjritiferous. Theiiresence 

 of graphite, native iron, and sulphurets in most aerolites, not to mention 

 the hydrocarbonaceous matters which they sometimes contain, tells us 

 in unmistakable language that these bodies come from a region where 

 vegetal)le life has performed a part not unlike that which still plays on 

 our globe, and even lead us to hope for the discovery in them of organic 

 forms which may give us some notion of life in other worlds than our 

 own. 



§20. Animal life has played in the chemical history of our planet a 

 part much less important than vegetation, since it is entirely depend- 

 ent for snpport upon the products elaborated for it by plants, and by 

 chemical forces. Thus, although many limestones are made up chiefly, 

 and even wholly of the calcareoas remains of marine animals, these 

 did no more than appropriate from the water the carbonate of lime 

 generated by the chemical actions explained in § 17. If the waters of 

 the present ocean do not deposit carbonate of lime, it is simply because 

 the amount of it now generated by the slow decomposition of the solitl 

 rocks is not more than is required for the living organisms which it con- 

 tains. Let these become extinct and the supply of carbonate of lime, 

 which would still continue, would soon cause deposits of precipitated 

 carbonate of lime. Such a condition of things existing in past ages, in 

 limited basins, has given rise to sediments of this kind, which constitute 

 some of the finest statuary marbles. 



The waters charged with the products of the sub-aerial decay of rocks, 

 convey to the sea, as we have seen, bicarbonates of alkalies, lime, and 

 magnesia ; but from the reaction of these on the chloride and sulphate of 

 calcium in the ocean waters carbonate of lime alone separates, since bi- 

 carbonate of magnesia decomposes chloride of calcium with formation 

 of magnesian chloride. When, however, in a closed sea-basin all of the 

 chloride of calcium is decomposed, the chloride of magnesium is attacked 

 by the alkaline carbonates, and the resulting carbonate of magnesia is 

 separated, mixed with the carbonate of lime which had accomjianied 

 these. 



When into a similar closed basin, or an evaporating salt lake in a dry 

 region, holding sulphate of magnesia, there is conveyed a water charged 

 with bicarbonate of lime, there results a double decomposition, giving 

 rise to sulphate of lime and bicarbonate of magnesia. The latter, being 

 the more soluble salt, remains dissolved, while the sulphate of lime crys- 

 tallizes out in the form of gypsum, but at a later period is deposited as a 

 hydrated carbonate of magnesia, generally mixed with carbonate of lime. 

 To effect this reaction it is necessary that there should be present such an 

 excess of carbonic acid as to keep the magnesia in the condition of bi- 

 carbonate until the gypsum has crystallized out, inasmuch as dissolved 

 sulphate of lime is readily decomposed by carbonate of magnesia. This 

 condition can only be attained by especial precautions in the atmosphere 

 of our period ; but by operating in an atmosphere more highly charged 

 with carbonic acid, the production of gypsum and magnesian carbonate 

 by this reaction is readily effected. We may hence conclude that it 

 was the more highly carbonated atmosphere of early iieriods which 



* Geology of Canada, 1863, p. 573. 



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