26 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 31, 



the aspect of a chemical precipitate than of a substance triturated 

 mechanically. In the clays and shales it is usually veiy uniformly 

 diffused through the mass ; in the sandstones and conglomerates it is 

 principally contained in the argillaceous matter which occupies the 

 interstices of the sand and pebbles, and it also stains the surfaces of 

 these fragments. In addition to the oxide of iron distributed through 

 the beds, there is in the fissures traversing them, a considerable quan- 

 tity of the same substance in the state of brown haematite and red 

 ochre, as if the colouring matter had been superabundant, or had been 

 in part removed and accumulated in these veins. Though the greater 

 part of the thickness of the lower carboniferous series consists of 

 reddish beds, there are many subordinate strata and minor groups of 

 beds from which the red oxide of iron is entirely absent. These 

 uncoloured beds are of three kinds. First, grey and dark sandstones 

 and shales, consisting of detrital matter similar to that of the red 

 beds. In some of these scarcely any ferruginous matter is present, 

 in others there are small quantities of the carbonate and sulphuret of 

 iron. Where these grey and dark beds appear in any considerable 

 thickness, they always contain either fossil plants, bituminous matter 

 or thin seams of coal, or all of these ; and even in thin and isolated 

 layers of this description, vegetable remains are often present. For 

 reasons to be stated in the sequel, I believe that the presence of this 

 organic matter is the cause of the absence of red colour in these cases, 

 and I am also disposed to extend the same explanation to certain 

 marly beds and blotched and variegated sandstones in which vege- 

 table matter does not appear. The second class of uncoloured beds 

 consists of limestones, of which there are several thick beds appearing 

 in a great number of places. These beds are very rarely coloured by 

 oxide of iron, and the few that are so contain also a little sand and 

 other detrital matter. Many of the limestones are made up of im- 

 broken shells and corals, others show under the microscope that they 

 consist of shelly fragments, and a few are laminated and crystalline, 

 and may have been deposited by water holding the bicarbonate of 

 lime in solution. Limestones of all these three kinds occasionally 

 contain bituminous matter. These beds of limestone certainly mark 

 long intervals in the deposition of detrital matter in the localities 

 where they occur ; and though the absence of red oxide of iron may 

 be in part due to the influence of putrefying organic matter, it also 

 indicates that the causes which produced the red colour were con- 

 nected with those which accumulated sand and other detritus, and 

 were not in active operation during those intervals when shells and 

 corals flourished. The third kind of beds destitute of red colour 

 consist of gypsum, which in this formation forms thick and conform- 

 able strata. These are generally very pure and colourless ; a few 

 however are blackened by bituminous matter, and I have seen one 

 containing sufficient red oxide of iron to give it a light flesh-colour. 

 The comparative absence of detrital matter from the gypsum, its con- 

 stant crystalline texture and its want of fossils, clearly indicate that 

 it is a chemical deposit ; and the same circumstances, in connection 

 with its regular stratification and association with marine limestones. 



