498 DR JOHN MURRAY AND MR ROBERT IRVINE ON THE 



these rocks the crystalline pyrites (FeS 2 ) has evidently its origin in the processes of death 

 and decay going on at the time of their deposition at the sea-bottom,* the sulphur of the 

 sulphide of iron being derived from the sulphates of the sea-water and not from the 

 sulphur of the organisms, as generally supposed. This decomposition seems to be 

 wholly due to the action of bacteria in causing putrefactive changes in dead organic 

 matter. We have found that if sea-water containing putrescible organic matter be 

 sterilised by boiling, and thereafter care be taken to prevent the ingress of bacteria to 

 this cooled liquid, the changes above indicated do not take place. Apparently the 

 organic matter must be broken down by bacteria into its component elements, which in 

 the nascent condition are capable of reducing the sulphates to a lower form of combina- 

 tion. The bisulphide of iron in the coal measures has without doubt a similar origin. 



In the water filtered from the Blue Muds we have found (see tables) the sulphur 

 present as sulphuric acid reduced from 25 to 50 per cent, of that originally present, the 

 quantity thus abstracted having been removed from the soluble condition of sulphates of 

 the alkalies or alkaline earths to the insoluble condition of sulphide of iron, and thus 

 permanently removed from the sea. 



It may be well here to take notice of the occurrence of Eed Muds or Clays which bulk 

 so largely in marine dej)osits. We have shown that the characteristic dark colour of 

 Blue Muds is due to the action of organic matter upon the sulphates in the sea- water and 

 the ultimate production of sulphide of iron with the iron of the deposit. In the Bed 

 Muds and Clays, either from the abundance of oxygen in the superincumbent waters, from 

 the ochreous matter present in the mud or clay, or from the organic matter being small 

 in quantity, the sulphide of iron is either not formed or is after formation soon oxidised 

 into ferric hydrate, which then gives its characteristic red colour to these deposits. A 

 parallel action to this may be observed in arable land, rich in peroxide of iron, the 

 organic matter in the soil and manure being rapidly oxidised at the expense of the per- 

 oxide of iron, which by this means is temporarily reduced to the condition of protoxide, 

 the iron acting as a carrier of oxygen from the air until the organic matter has been 

 oxidised into carbonic acid. It may be accepted as the rule that muds containing a 

 large amount of organic matter relatively to the iron present invariably partake of the 

 characteristic blue-black colour, whilst if organic matter be low in amount, or altogether 

 absent, the black sulphide is either not formed at all, or is oxidised into peroxide of iron. 

 An example of the latter condition is shown in the mud deposited by the Amazon and 

 other Brazilian rivers, and also by the rivers that pour their waters into the Yellow Sea. 

 These contain a large quantity of ferruginous matter, in amount far exceeding the organic 

 matter present, the consequence being that the mud when deposited on the sea-floor 

 retains the characteristic brown-red colour of deep-sea Red Clays even near the shore. 



* Ferrous sulphide, to which Blue Muds mainly owe their deep black colour, gradually becomes converted into 

 ferric sulphide as these muds harden into shales and schists. During this action or change of condition part of the iron 

 of the ferrous sulphide probably is oxidised, the sulphur set free either combining with the ferrous sulphide to form 

 ferric sulphide, or a portion of the ferrous sulphide in the Blue Muds may be changed by oxidation into ferric sulphate, 

 which, in the presence of organic matter, may become reduced to ferrous sulphide and free sulphur, thus providing 

 material for the formation of the iron pyrites (FeS 2 ) so commonly associated with shales, schists, slates, &c. 



