226 THE BUILDING OF THE DERBYSHIRE LIMESTONE. 



Globigerina ooze is replaced by one made up almost entirely of the 

 frustrules of diatoms, while from below 2,000 fathoms, silicious 

 oozes, composed of radiolaria, are brought up. From still greater 

 depths (over 3,000 fathoms), a red clay, devoid of organisms, is 

 brought up. None of the pelagic deposits find analogues in the 

 mountain limestone, and it would appear that the sea in which it 

 was formed never attained such depths. 



If we trace the limits of the limestone area in different directions, 

 we are able to make out the probable limits of the sea in 

 which it was laid down, and thus get an idea of the physical 

 geography at the period of its formation. If we trace the Derby- 

 shire limestone westward into Wales, we find that it gradually 

 becomes thinner, and associated with it are beds of sandstone and 

 shale. This is a sign of shallow water, and indicates the nearness 

 of an ancient coast-line. If we follow it to its junction with older 

 rocks, we find it resting unconformably upon them. It rests 

 against the metamorphic rocks of Anglesey ; in Shropshire, North 

 Wales, Westmorland, etc., upon Silurian rocks ; while in many 

 localities it conformably overlies beds of a deep red colour, which 

 were early referred to the Old Red Sandstone. It would appear, 

 however, that these intermediate rocks, which belong to the 

 system known as the Devonian, represent various periods of the 

 long interval which elapsed between the close of the Silurian and 

 the beginning of the Carboniferous periods. These beds repre- 

 sent the shore-line of the sea in which the carboniferous limestone 

 was deposited. Following the limestone northwards, we trace the 

 ancient beach passing under Ingleborough, and in the dales cut 

 into the mountain may be seen the lowest beds of the 

 carboniferous system, the lower ones consisting of angular 

 fragments derived from Silurian rocks, and passing upwards into 

 beds containing large water-worn pebbles, and these again into a 

 calcareous sandstone. Tracing it northwards and eastwards into 

 Durham and Northumberland, we find intercalated with the 

 calcareous strata beds of shale, sandstone, and coal, indicating the 

 existence of land in that direction. In Ireland the lower 

 carboniferous sea overspread the greater portion of the country, 



