result of external denudation, and deposited these in stratified 

 order with the multitudes of Molkiscs, Cepholopods and Saurians 

 which lived and died near the spots where their remains are now 

 found, whilst at the same time a chemical work of cementation 

 was taking place, producing under varying circumstances those 

 wonderful alternations of shale and stone which are exhibited in 

 many a section of the Lias. In such sections a few feet of Shale 

 composed of exceedingly thin layers, "leaf-like laminae" as 

 Hugh Millek styles them, so friable as to break and crumble on 

 the slightest pressure, are succeeded by a band of a very compact 

 Crystalline Limestone, this by Shales, and these again by stone — 

 it may be less crystalline, but still very compact. To account 

 for such varying results in the deposition of Carbonate of Lime, 

 what a constant recurrence of varying agencies, mechanical and 

 chemical, must have prevailed. 



If in the study of this subject it is attempted to trace out the 

 possible history of the atoms of Calcium, which are found in some 

 specimen of recent Tufa, what an illustration may it afPord of 

 the wonderful story of the changes which have taken place in 

 the crust of the globe. For instance, a spray of moss recently 

 incrusted by Carbonate of Lime, the atoms of Calcium in which 

 have just now been extracted by water from their long rest in 

 the Oolite beds, where they have lain during the vast lapse of 

 time since those strata were dej)0sited, time which has sufficed 

 for the formation of the upper members of the Oolitic system 

 and the Wealden, and all the members of the Cretaceous and 

 Tertiary systems, and for those great superficial changes which 

 have taken place during the Quarternary period. And what 

 parts may these atoms of Calcium have played before they attained 

 their late position in elevated beds of Oolite ? Some, perchance, 

 as they floated in the waters of the old Oolitic ocean, may have 

 been secreted by a limpet, and appropriated to the enlai-gement 

 of the shell which protected and sheltered that ancient Mollusc, 

 as it glided over the submerged rocks, or lay firmly adherent to 

 exposed rocks on the shore, waiting the re-flow of the tide. The 

 shell after the death of its constructor, after being tossed hither 

 and thither by the waves, its prominent edges worn ofi" by striking 



