72 report — 1859. 



Carbonate of lime 76*20 



Carbonate of magnesia traces 



Clay 8-30 



Sand 14-65 



Carbonaceous matter, pyrites and amorphous sulphide \ „,„ 



of iron J 



99-65 



This limestone, which is very hard and has a conchoidal fracture, owes 

 the physical properties which distinguish it to the sandy residue forming 

 its skeleton. The latter retains the external form and appearance of the 

 original rock ; and even fossil-casts may be recognized and determined. If 

 the skeleton be ignited, the carbonaceous matter is burned away, leaving the 

 cast of the fossil perfect. The skeleton thus exposed represents the sand of 

 the original sea-bottom prior to its infiltration with calcareous matter. 



When the quantity of argillaceous matter equals that of the carbonate of 

 lime, and especially when the carbonaceous matter is present in a consider- 

 able quantity, this calcareous residue presents all the character of a true 

 mud. This is the case with the lower limestone shales from the neighbour- 

 hood of Drogheda, which may be represented by the following composition : — 



Carbonate of lime with carbonate of magnesia 47*10 



Residue of clay and sand containing iron pyrites .... 47'75 

 Carbonaceous matter 5*15 



100-00 

 This residue, and indeed all the similar beds belonging to that formation, 

 contain a good deal of iron pyrites and sulphide of iron in what may be 

 called an amorphous state — apparently a proto-sulphide, as it evolved 

 sulphuretted hydrogen on being treated with acids; both these are included 

 in the clay and sand, and partly in the carbonaceous matter. 



I may here observe that the quantity of residue, and of carbonaceous 

 matter, varies in different parts of the rock from the same quarry. Thus a 

 specimen taken at a short distance from the locality of the last specimen had 

 the following composition : — 



Carbonate of lime with carbonate of magnesia 55*40 



Residue I Clay and sand with Py ritcs S6 ' 00 



\ Carbonaceous matter 8*60 



100-00 

 In some of these consolidated muds the lime is almost wholly absent ; the 

 lithological character of the residues, however, remains constant. The sand 

 separated from the clay is extremely fine when observed under a strong lens. 

 The analysis of some of these residues may perhaps serve to trace the source 

 from which they were derived. 



I have already alluded to the existence of two compounds of iron with 

 sulphur in these beds; and I may here remark that the characters which the 

 lower limestone shales, as for instance that of Drogheda, present, appear to 

 offer an explanation of the circumstances under which these sulphides were 

 formed. We are daily witnesses of the fact, that under the influence of water 

 and organic matter, and exclusion of air, sulphates dissolved in water are 

 reduced to the state of sulphides, which convert the salts of iron in contact 

 with them into sulphide. The sulphide thus formed is amorphous, as may 

 be observed in the black mud which is found under the pavement of streets, 

 and which evolves sulphuretted hydrogen. Now the mud from which these 



