TIHE ORIGIN OF RED BEDS 241 



as most terrigenous red muds lose their color on entering the sea. 

 A vivid description of this process of loss of color in the case of 

 certain rivers in Nova Scotia is given by Dawson, as follows: 



This harbour [Pictou] receives 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 shore 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 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 .... which grows abundantly on the mud flats In some parts 



of the deposit forming in Pictou harbour, the vegetable matter which caused 

 the change of colour is so completely decomposed that no visible fragments 

 of it remain. 1 



The chemical action of marine organic matter is summarized 

 by Clarke in part as follows : " Decomposing organic matter reduces 

 the sulphates of sea-water to sulphides, which by reaction with 

 carbonic acid yield sulphuretted hydrogen. Bacteria also assist 

 in the process." 2 



The interbedding of marine limestones with Red Beds shales 

 in the Texan section is consistent with an origin for the shales 

 similar to that of the semi-oceanic sediments from the Amazon 

 River. The interbedding of gray and green strata with red ones 

 in certain of the Red Beds series indicates an oscillation of domi- 

 nance between oxidizing and deoxidizing conditions, such as might 

 be caused at the margin of the sea or in the waters of an inclosed 

 basin by variations in the rate of sedimentation or in the abundance 

 of organic matter. The marine type of deposition of red sediments 

 is not to be neglected, therefore, in an attempt at reconstruction 

 of the conditions of origin of the Red Beds; though the complete 

 absence of marine fossils from other parts of this group of sediments, 

 together with independent proof of the continental origin of most 

 of the group, shows that the marine type cannot be of more than 

 subordinate importance. 



1 J. W. Dawson, "On the Colouring Matter of Red Sandstones and of Greyish 

 and White Beds Associated with Them," Quar. Jour. Geol. Soc. London, V (1848), 29. 



- F. W. Clarke, "The Data of Geochemistry," 2d ed., U. S. Geol. Survey Bull. 

 No. 491, iqii, pp. 136-137. 



