236 PROCEEDINGS OP THE 



and fortunately he was tempted to collect a quantity of this 

 unpromising-looking clay from the cavities. After being dried and 

 washed it was found to be moderately fossiliferous, containing a 

 group of sponge spicules of varied forms new to our Carboniferous 

 strata, as well as various small molluscan remains and Conodonts, 

 several also new. These sponge spicules and shells are of a fine 

 creamy-white colour, and, as formerly stated, are generally in an 

 excellent state of preservation. Indeed, if one did not know the 

 genera or species to which they belong, he would never suppose 

 that they had been obtained from so old a formation as that of the 

 Carboniferous limestone, for they look more like a group of organ- 

 isms from recent Tertiary strata, such as those of the Paris basin. 



In explanation of the peculiar conditions under which they 

 are found, it has been conjectured that the cavities now filled with 

 clay were originally calcareous nodules, which had been segregated 

 in the calcareous shale by the drawing together of the lime of 

 the deposit into nodular masses through chemical attraction while 

 the rock was still in a soft condition, and after the same manner in 

 which it is believed nodules of clay-ironstone, and nodules of flint 

 were formed.* These calcareous nodules would enclose the organ- 

 isms lying in the stratum wherever they were segregated, while the 

 lime present in their composition would, perhaps, tend to the better 

 preservation of the organisms than in other portions of the bed. 

 Be this as it may, we now find that it is only the organisms 

 enclosed within the walls of the cavities originally filled by the 

 nodules which are now well preserved ; those found in other por- 

 tions of the stone being all in the form of casts, or so firmly com- 

 bined with the rocky matrix that they cannot be extracted. 



In the lapse of time these beds were brought to the surface of the 

 ground through denudation of the overlying strata, the calcareous 

 matter was probably removed by the passage through the beds 

 of water containing carbonic acid, so that the lime originally form- 

 ing the nodules was entirely dissolved away, leaving behind only 

 the clay formerly combined with it, and the organisms at the 

 same time were left in such a state of freedom in the argillaceous 



* Recently, I have found calcareous nodules, enclosing similar fossils, in a 

 bed of shale in the upper limestone series of the same neighbourhood, and 

 probably on the same geological horizon, but there the strata have not suffered 

 from the same corroding action as that which has robbed the Glencart beds of 

 all their lime. 



