224 THE BUILDING OF THE DERBYSHIRE LIMESTONE. 



of our own limestone will, on investigation, )ield interesting 

 results. 



In the upper limestones of Derbyshire we find interslratified 

 layers and nodules of a flinty deposit called chert, the origin of 

 which forms an interesting geological problem. Frequently it 

 occurs in large masses, and contains silicious casts of the organic 

 remains it has replaced. Often these silicious pseudomorphs 

 retain the minute structure of the original object, which has 

 evidently been replaced particle by particle as it was removed, in 

 the manner in which silicification of wood gives rise to the 

 formation of wood opal with all the delicate structure of the 

 original matter. These chert bands occur in the upper 

 carboniferous limestone of Ireland, and formed the subject of a 

 paper by Messrs. Hull and Hardman.* It may be well to point 

 out that silica occurs not only in the crystalline condition, in 

 which it is insoluble, but also in a colloid condition, in which 

 state it is to some extent soluble in water. Microscopical exami- 

 nation of chert tends to show that its silica is colloid. Professor 

 Hull, in the paper referred to, came to the conclusion "that 

 carbonate limestone chert is essentially a pseudomorphic rock, 

 consisting of gelatinous silica replacing limestone of organic 

 origin, cliiefly foraminiferal, crinoidal and coralline," and that the 

 replacement was not of organic origin. He regarded it as 

 probable that the coralline and crinoidal beds would, after their 

 formation, be porous and open. The sea- water, containing 

 dissolved silica, would percolate through this, and, since it has 

 been shown that mineral or organic objects formed of 

 carbonate of lime are liable to be replaced by silica when 

 submerged in water in which this mineral is dissolved, the 

 replacement of calcium carbonate by silica would follow as a 

 purely chemical process. Subsequently, Dr. Hinde brought 

 forward evidence to show that the silica had been derived from 

 the silica of sponge remains. 



It has already been pointed out that the organic remains met 



* " Scientific Transactions of the Royal Dublin Society," Vol. I. (New 

 Series). 



