534 PHYSICAL GEOGRAPHY. 



the sulphate is reduced to a sulphuret. Gold is not known in nature as a 

 sulphuret, though commonly diffused through pyrites ; hence it is pre- 

 sumed that the precious metal was precipitated simultaneously with the 

 deoxidation of the sulphates, not passing through the intermediate con- 

 dition of a sulphuret. The deoxidizing agent was probably vegetation. 



The accumulation of the large beds of iron and copper pyrites in 

 Gardner's mountain was brought about at the same time, and under 

 similar conditions. For the formation of sulphurets we require origi- 

 nally a sulphate ocean, just as in the case of gold. The deoxidizing 

 action of vegetation will reduce the sulphates to sulphurets; and thus 

 deposits of pyrites of enormous extent may be produced, provided there 

 is a sufficient store of the iron and sulphur. In natural waters, contain- 

 ing sulphates of lime and magnesia, the process of sulphate reduction 

 in the presence of decaying plants may be observed at the present day ; 

 and, if carbonic acid be present, sulphuretted hydrogen gas is also 

 evolved. The same principle is made use of in analytical laboratories 

 for the separation of metallic sulphurets. A stream of sulphuretted 

 hydrogen is passed into metallic solutions, when the element readily 

 combines with the sulphur, producing an insoluble sulphuret, which is 

 precipitated. The copper pyrites is so closely connected with the iron 

 that it has probably been accumulated in the same way. In the Ammo- 

 noosuc gold region copper is very common as an impurity in the strata 

 at localities remote from valuable beds of ore; so that the sulphate 

 ocean must have been highly charged with the mineral solutions. The 

 formation of the lead sulphuret may be described in similar terms. 



By referring to an argument set forth above for the existence of veg- 

 etation, it will appear that the formation of the iron oxides is akin to that 

 of the sulphurets. Vegetation is the agent required at the present day 

 for the production of both ; hence, by looking at the process in another 

 way, we may derive a different conclusion. The ores of iron and copper 

 require, at the present day, the presence of vegetable matter for their 

 formation. These ores accumulated largely in the Huronian age ; hence 

 it is probable that plants existed at this time. No other evidences of 

 their existence have yet been discovered. 



There are three divisions of this age: First, the period of the deposi- 

 tion of the copper and iron beds, with an auriferous tinge. Second, the 



