MARSHES. 25 



This singular and complicated series of processes, into all the details 

 of which I have not entered, is of especial interest to the geologist, 

 as it explains the causes which have produced the gray colour and 

 abundance of sulphuret of iron observed in many ancient rocks, which, 

 like the blue marsh, have been produced from red sediment, changed 

 in colour by the presence of organic matter. It also explains the 

 origin of those singular stains Avhich, in rocks coloured by iron, so 

 often accompany organic remains, or testify to the former existence of 

 those which have passed away. It farther shows the reason of the 

 paucity of organic remains in red rocks, for the red oxide of iron, when 

 jiresent in excess, tends to corrode and destroy any organic matter 

 which may be present ; and on the other hand, an excess of organic 

 matter tends to deoxidise the iron and remove it in a state of solution, 

 or change it into a sulphuret, according to circumstances, — the colour 

 of the sediment being changed in either case. 



Much geological interest attaches to the marine alluvium of the Bay 

 of Fundy, from the great breadth of it laid bare at low tide, and the 

 facilities which it in consequence affords for the study of sun-cracks, 

 impressions of rain-drops, foot-prints of animals, and other appearances 

 which we find imitated on many ancient rocks. The genuineness of 

 these ancient traces, as well as their mode of preservation, can be 

 illustrated and proved only by the study of modern deposits. I 

 quote a summary of facts of this kind from a paper on rain-prints 

 by Sir Charles Lyell, who was the first to direct attention to these 

 phenomena as exhibited in the Bay of Fundy .'^" 



" The sediment with which the waters are charged is extremely fine, 

 being derived from the destruction of cliffs of red sandstone and shale, 

 belonging chiefly to the coal measures. On the borders of even the 

 smallest estuaries communicating with a bay, in which the tides rise 

 sixty feet and upwards, large areas are laid dry for nearly a fortnight 

 between the spring and neap tides, and the mud is then baked in 

 summer by a hot sun, so that it becomes solidified and traversed by 

 cracks caused by shrinkage. Portions of the hardened mud may then 

 be taken up and removed without injury. On examining the edges of 

 each slab, we observe numerous layers, formed by successive tides, 

 usually very thin, sometimes only one-tenth of an inch thick, — of un- 

 equal thickness, however, because, according to Dr Webster, the night- 

 tides rising a foot higher than the day-tides throw down more sediment. 

 When a shower of rain falls, the highest portion of the mud-covered 

 flat is usually too hard to receive any impressions ; while that recently 

 uncovered by the tide, near the water's edge, is too soft. Between 

 * Journal of London Geological Society, vol. vii. p. 239. 



