90 PALAEOZOIC TIME. [CHAP. VI. 



formed, mainly of mud and sand, though every now and then 

 calcareous animals established themselves in sufficient num- 

 bers to give rise to beds of limestone. At a certain distance 

 from the shore all the sediment sank down to the bottom, 

 and beyond this limit the water was bright and clear, and 

 the only deposit consisted of accumulations of the hard, 

 calcareous parts of marine animals which are now pure 

 limestone. 



" In the deep hollows the deposits of limestone reached 

 a great thickness ; over the ridges which parted the hollows 

 it was not so thick. This will explain how it is that the 

 Carboniferous Limestone shows such great variations in 

 thickness at different spots. The growth of the limestone 

 gradually filled up the deeper parts of the sea, and at last 

 the area became as shallow throughout as it had been 

 originally only at its edges. The mixed deposits of sand- 

 stone, shale, and impure limestone, which had at first been 

 confined to the neighbourhood of the shore, now extended 

 themselves over nearly the whole marine tract, and the 

 deposition of the Toredale rocks began 



" After a time a further important change took place. 

 Either by the outlets becoming blocked up, or by the 

 upheaval of a portion of the bottom, the land-locked area 

 in which the lower marine portion of the Carboniferous 

 rocks was deposited became cut off from communication 

 with the open ocean .... and converted into a fresh- 

 water lake or a large estuary which received the waters 

 of rivers flowing from the north, east, and south. 1 In 

 this freshwater, or brackish area, the Millstone Grit and 

 Coal-measures were deposited. When the water was at its 

 shallowest, currents piled up banks of sand, or drifted coarse 

 materials over the bottom, and shot them down into hollows, 



1 It would be better described as an immense delta or fenland, in- 

 cluding many large lagoons and wide channels, surrounded by swamps 

 which were never much above the level of the sea. 



