1848.] DAWSON ON COLOURING MATTER OF RED SANDSTONE. 29 



speedily find its way to the sea, or might even in some eases be pro- 

 duced in its bottom, and coming into contact with calcareous matter 

 accumulated by shell-fish and madrepores, would be deposited, in com- 

 bination with lime, in the form of gypsum. There might thus in 

 the seas of the carboniferous period be alternations of organic accu- 

 mulation and detrital and chemical deposition, producing a formation 

 precisely corresponding with the lower carboniferous series of Nova 

 Scotia, as described in the beginning of this paper. 



I do not so far overrate the force of the above remarks, as to sup- 

 pose that they prove that the oxidation of iron pyrites has been the 

 sole cause of the red colours of sedimentary deposits. They may 

 however lead geologists to inquire if any production of red oxide of 

 iron attends the formation of sulphuric acid in modern volcanic re- 

 gions, — if in other countries the first appearance of red sandstones and 

 shales is attended with the presence of g}^sum or other sulphates, and 

 if the materials of the red beds have been derived from rocks con- 

 taining iron pyrites. If these circumstances are of general occurrence, 

 they may perhaps show that the cause above referred to is also 

 general. 



In the lower carboniferous series of Nova Scotia, there are, as be- 

 fore stated, grey, dark and white beds interstratified with red rocks 

 forming the mass of the deposit ; and though the sediment forming 

 these has no doubt in many instances been originally uncoloured, 

 there are other instances in which they appear to have consisted of 

 red sediment deprived of its colour by chemical agents after its depo- 

 sition. This may have been effected by the agency of organic matter 

 in two ways, the first of which applies more especially to marine, the 

 second to freshwater deposits. 



The first consists in a reversal of the process above described, or 

 in the conversion of oxide of iron into sulphuret of the metal. I 

 shortly referred to this change in a paper sent to the Geological So- 

 ciety in 1845, but did not state the facts on which my views were 

 founded. My attention was first directed to this process by observing 

 it actually in progress in the harbour of Pictou. This harbour re- 

 ceives the waters of three rivers and several smaller streams, which 

 in times of flood carry into it large quantities of reddish mud, which 

 sometimes discolours the whole surface. This mud, with similar sedi- 

 ment from the shores of the harbour, is deposited in the bottom, and 

 there undergoes a remarkable change of colour. A portion of old 

 mud recently taken from the bottom is of a dark grey colour, and 

 emits a strong smell of sulphuretted hydrogen. When dried it loses 

 this odour, and its colour is a pure grey without any trace of red. 

 If a piece of the dried mud be heated to incipient redness, it emits a 

 sulphurous odour, and at once resumes the red colour which belonged 

 to the sediment before it was deposited. It thus appears that the 

 iron of the red clay has entered into combination with sulphur, and 

 this is probably obtained from the sulphates contained in the sea- 

 water, by the deoxidizing influence of decaying vegetable matter, the 

 greater part of which seems to be furnished by the eel grass (Zostera 

 mai'ina), which grows abundantly on the mud flats. It is evident 



