366 pitocEEDEs'GS 01? xhe geological societt. [March 12, 



not, and subject to be overflowed now and then, and to have vege- 

 table matter left in them. The reason why the sediments in which 

 these beds occur are not ferruginous or calcareous to the same 

 extent becomes then equally plain, because the bed of Clay-iron- 

 stone formed would sometimes be sufficient to fill the hollow, or a 

 slight subsidence might carry that terrestrial horizon down. 



Sometimes these horizons appear to have been spreads of warp, 

 as in the following section copied from the Explanation of Quarter- 

 sheet 93 S.W. In it the black shale is coloured by carbonaceous 

 matter, and in fact is an impure drift coal, aud therefore marks a 

 succession of terrestrial surfaces. 



No. 3. 



ft. in. 



Ironstone (impure) 1 



Black shale.... 9 



Ironstone Oh 



Black shale 2" 



Ironstone 1 



Black shale 8 



Ironstone 1£ 



Black shale l" 



Ironstone, Top Balls 2£ 



Black shale 6 



Ironstone, Middle Balls 2 



Black shale 11 



Ironstone, Low Measures 1-J- 



Blackshale 1 



The above section rests upon the terrestrial surface of the Black- 

 Bed Coal. 



I can imagine nothing more convincing as to the terrestrial origin 

 of these beds than the above section. By " terrestrial " I mean a 

 surface exposed to the air, but subject to be covered by floods. 



The process described by Mr. Hunt might then come into play; 

 in fact I cannot see that it could avoid doing so. Carbonic acid 

 formed in the lagoons from decomposing vegetable matter, meeting 

 with protoxide of iron in solution, would unite with it and form 

 a carbonate of iron ; and this, with the mud in the lagoon, would form 

 a clay-ironstone. 



I may remark that it cannot be shown that the chemical agents 

 have not formed beds of clay-ironstone when discharged into open 

 waters, that it cannot be proved that they ever did so under those 

 circumstances, and that the above sections do prove that clay- 

 ironstone beds have been formed on terrestrial surfaces. 



If the action were possible in open waters, why are the beds not 

 more generally diffused '? and why are they so compact and local 

 amongst the beds in which they lie ? Surely the red marls of Prof. 

 Bamsay's inland seas should contain them; but they do not. 



The above supposition will explain all the varieties of Clay- 

 ironstone lying in beds, of course not those formed by segregation 

 from a state of general dispersion through the sediment in which 

 they lie as concretions. Such concretions may, however, be made 



