IxXXviii PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



ness of the mass, as it appears from measuring the beds vertically to 

 their outcrop, there are more than 1500 cubic miles of, chiefly, red- 

 coloured detrital matter. These beds afford, comparatively, few ani- 

 mal remains, and those chiefly in the lower portion, while below them 

 are a series of strata often teeming with organic remains. No sooner 

 did these red-stained beds cease to be deposited than marine creatures 

 again resorted to the waters above them, as we see by the carboniferous 

 limestone that rests upon the old red sandstone. A great change of 

 physical conditions therefore must have taken place from the com- 

 mencement of these red beds. Peroxide of iron mechanically sus- 

 pended in rivers is known to be fatal to the animals previously living 

 in them. We can thus account for their great rarity both in the old 

 red and new red sandstone series ; but as yet nothing has been discovered 

 to lead us even to conjecture whence this enormous quantity of iron 

 in deposits formed out of the detritus of pre-existing rocks has been 

 derived. Analyses show that marls of the old red sandstone have 

 yielded 6 per cent, of peroxide of iron, and those of the new red 

 nearly 10 of peroxide and 4^ of protoxide. But there is a well-known 

 character common to both, that which is the origin of the term poi- 

 cilitic as applied to the new red sandstone, viz. the prevalence of 

 layers, stripes and patches of a bluish-green and grey colour ; the cause 

 of which has hitherto been a matter of great difficulty to account for. 

 Sir H. de la Beche states that Captain James, R.E., has pointed out 

 the probability that the clefts and joints of the rocks are changed from 

 red to bluish-green by the percolation of water charged with vegetable 

 matter, which, under certain conditions, changes the peroxide into a 

 protoxide*. " With the knowledge," he says, " that under the condi- 

 tions where vegetable acids are forming in contact with peroxide of 

 iron, the latter is robbed of part of its oxygen, and converted into a 

 protoxide, it is interesting to consider if the colours of these greenish 

 bands (often very marked, and continuing over the same planes amid 

 the red rocks for considerable areas, showing the operation of some com- 

 mon and widely-spread contemporaneous cause over them,) may not 

 be due to a change produced upon the peroxide of iron by vegetable 

 matter. Throughout the Silurian rocks, and above the black slates 

 (formerly black mud), wherein carbon is found, we cannot suppose 

 the animals whose remains are often so abundantly entombed in the 

 mud, silt and sand of the time, to have existed without marine vegeta- 

 tion, though we commonly find no trace of it, the vegetation with the 

 soft parts of the animals having been decomposed, partly into acids, 

 and partly into different gases, which if coming into contact with 

 peroxide of iron would convert it into protoxide," the state in which 

 it exists in the bluish-green parts of the sandstones. Some analyses 

 of the blue marl, conducted in the laboratory of the Museum of Eco- 

 nomic Geology, led Dr. Lyon Playfair to remark that the carbonic 

 acid is greater in the blue than in the red marl, because the carbonic 



* Since this Address was delivered, I have been reminded that Mr. J. Dawson 

 of Pictou, in a paper read before this Society 22nd January, 1845, suggested that 

 the bleaching of the red-sandstone may have been caused by the decomposition of 

 vegetable matter. — Quart. Jour, of Geol. Soc. i. 327. 



