1899O FoORD.^- Carboniferous Brachiopoda and Mollusca. 69 



period in which they originated. Thus, the limestone repre- 

 sents the sediments, almost wholly organic, deposited in a 

 great open sea, like a Mediterranean, bordered by the older 

 Palaeozoic rocks and extending from the west of Ireland, 

 through England, the north of France and Belgium to 

 Westphalia. This Carboniferous sea covered large areas in 

 Russia also, and, going outside Europe, in Africa, North 

 America, China, and Australia. 



Subsequently, during the latter part of the Carboniferous 

 Period (I am here referring to the whole system) new 

 physical conditions set in, the sea became shallower, and 

 sandy and muddy deposits were accumulated by the waste of 

 the neighbouring land ; lagoons were formed in the more 

 contracted area, and, under favourable climatic conditions, 

 a luxuriant marsh vegetation sprang up upon the newly 

 elevated land. Then there came a subsidence of the land, 

 the forests were submerged and sandy detritus spread over 

 them, and this process of elevation and subsidence was 

 repeated again and again till immense thicknesses of alternat- 

 ing beds of sandstones, shales, and coal seams were formed. 

 These events required of course a vast period of time for their 

 accomplishment. In endeavouring to account for such a 

 great thickness of sedimentary rock as that of the Carboni- 

 ferous Limestone, we must again invoke earth-movements on 

 a large scale ; its accumulation (it exceeds 3,000 ft. in thick- 

 ness in the south-west of Ireland) can only be explained on 

 the assumption that the oceanic area in which it was 

 deposited was a gradually and slowly subsiding one, and that 

 the process of subsidence was greatly prolonged, since other- 

 wise so considerable a thickness of homogeneous rock could 

 not have been deposited. Movements involving the sub- 

 sidence and elevation of the land are attributed to different 

 events, all having their origin in the constitution of the 

 interior of the earth. One of these is the secular contraction 

 of the earth's crust in the process of cooling from its primitive 

 molten state, another is that the weight of a vast quantity of 

 sediment derived from the waste of the land may have caused 

 a sinking of the area to which it was conveyed, while a 

 corresponding elevation would take place in the one from 

 which it was removed. Proofs of earth-movements are to be 



