285 



contain little or no carbonaceous matter, the marks of innumer- 

 able roots of Stigmaria, and parts of other plants which every- 

 where penetrate the mass, show that at one time they must have 

 been crowded with vegetable remains. 



A further and yet more striking proof of the influence which 

 the contiguous vegetable matter has had, in the formation of 

 the Proto-Carbonate, is seen in the fact, that the most productive 

 layers of the ore are commonly met with quite near to the beds 

 of coal, and that frequently courses of the nodules are found in 

 the carbonaceous shales or partings which lie in the midst of the 

 seam itself. 



While the strata including the Proto-Carbonate are thus 

 distinguished by the admixture of more or less carbonaceous 

 matter, they are also remarkable for seldom exhibiting a dis- 

 tinctly red tint. Presenting, where not weathered, various 

 shades of greenish gray and olive and bluish black, they only 

 become brown or red where, by exposure to the air, the Proto- 

 Carbonate has been converted into the Sesquioxide of Iron. On 

 the other hand, those divisions of the coal measures which have 

 been but slightly charged with vegetable matter, as for example 

 the barren shales of the Serai Coal rocks before alluded to, 

 contain much red material, both in distinct strata and mottling 

 the general mass, and are throughout more or less impregnated 

 with the Sesquioxide. 



A like general law as to color would seem to apply to the 

 other great groups of sedimentary rocks, which include in 

 particular beds accumulations of vegetable or other organic 

 exuvise. Thus, in the New and Old Red Sandstone formations, 

 which generally include so large a proportion of sediment 

 colored by the red Oxide of Iron, organic remains are of com- 

 paratively rare occurrence, and when present are met with 

 almost exclusively in the gray and olive and dark-colored strata 

 which are interpolated in certain parts of the great masses of red 

 material. This relation is beautifully shown in the middle 

 secondary rocks of the Atlantic slope, which extend in a pro- 

 longed belt from the Connecticut Valley into the State of South 

 Carolina. In the strata of red sandstone and shale, which form 

 the chief part of the mass, vegetable or animal exuvise are almost 



