25 



amount of movement ; they would be subject to tbe chemical aud 

 molecular changes which the pressure resulting from a great 

 depth of water would help to being about, the affinities it would 

 call into play, the solutions it would facilitate. 



The Hypothesis op their Formation op Flint since the 

 Upheaval and Solidipication of the Chalk. 

 The argument for this is well put by Professor Green in his 

 "Geology for Students" (page 173). Balls, lumps, or nodules 

 of different composition from the rocks in which they are found, 

 are common in many rocks. That such have in many cases been 

 formed since the upheaval and solidification of the rock is shown 

 by the lines of stratification running through the nodules. A 

 shell, plant, fish, or even a grain of sand has served as a nucleus 

 round which the nodules have consolidated and has even deter- 

 mined their shape. The matter of which these nodules consist 

 must have been disseminated through the rock, but Professor 

 Green candidly admits that he speaks of concretionary action, as 

 he terms it, as being the cause of the formation of these nodules, 

 he has no explanation to offer of what this action really is. It is 

 alleged on good authority, however, that when flints have been 

 ground up into powder and mixed with clay for pottery purposes, 

 the silica after a time separates out aud forms incipient concre- 

 tions of flint. Aud this is not the only instance of such segrega- 

 tion that has been observed. Such facts may be adduced, they 

 may illustrate, but do not explain, the sporadic nodules of flint in 

 the chalk, and still less the layers of them, or the thick tabular 

 sheets which in some places take their place. 



If it be asked why these flinty nodules occur in such regular 

 layers parallel to the planes of bedding, it must be admitted that 

 neither of the hypotheses just reviewed can furnish an adequate 

 explanation. 



Few researches have been made of late years more impoi'taut 

 than those bearing ou the amount of colloid or soluble silica still 

 found disseminated through certain rocks. In the chalk of 

 Collingbourne, Messrs. Jakes-Brown, and Hill found no less than 

 38'69 per cent. (On the Lower Chalk of Berkshire and Wiltshire). 



They say further " One bed contains very definite siliceouB 

 concretions of irregular shapes comparable to nodules of flint, 

 but unlike such nodules in being so closely united with the sur- 

 rounding chalk that they are not readily separated from it." 

 The colloid silica which they contain to so large an extent is in 



