THE BUILDING OF THE DERBYSHIRE LIMESTONE. 225 



with in the Derbyshire limestone, closely resemble the deposits 

 which are forming in modern seas. We find analogues to the 

 shelly, crinoidal, and coralline deposits ; but, while we find 

 numerous foraminifera, extensive deposits such as the chalk or 

 the modern Globigerina ooze are conspicuous by their absence. 

 Nor do we find anything at all comparable to the silicious oozes 

 which are being formed of the shells of radiolaria and other 

 silica-secreting organisms. It is possible that the silica of the 

 chert may have been partly derived from such deposits, if they 

 ever existed, but this seems doubtful. The question is a deeply 

 interesting one,_and will probably repay further investigation. 



Let us now endeavour to call up to our imagination the 

 physical conditions under which this limestone was formed. It 

 has already been stated that in Derbyshire its base has never been 

 reached ; nevertheless, it has been calculated, from the measured 

 geological sections, that it is not less than 5,500 feet in thickness. 

 The whole of this is a marine deposit of great purity, and 

 therefore must have been formed in a fairly deep sea, to which 

 sedimentary matter from the land rarely found its way. We also 

 infer, from its great thickness, that while it was being formed, 

 subsidence was taking place. We are driven to this conclusion by 

 the fact that in this thick deposit the fossils in the upper beds do 

 not differ from those in the lower ones to such a degree as would 

 be the case if the gradual accumulation of the deposit had 

 involved a shallowing to the extent of 5,500 feet. Differences do 

 occur, but we look in vain for such a change in the fauna as 

 would be brought about by a shallowing of the sea to the extent 

 of the thickness of the marine deposits. While we may regard 

 the mountain limestone as a deep sea deposit, we are forbidden 

 to imagine that its depth approached that of the deeper parts of 

 the Atlantic. Soundings, made in the deeper parts of the Atlantic, 

 reveal the fact that oozes are being formed of the calcareous and 

 silicious shells of minute organisms which inhabit the surface and 

 bottom waters. From a depth of about 2,000 fathoms in the South 

 Atlantic has been brought up an ooze made up of globigerina, 

 orbulina, coccoliths, rhabdoliths, etc. In some places this 

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