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MINERALOGY. 



[Sect. VIII. 



of the cavity, others have accumulated in flat layers 

 generally considered to have been formed horizontally. 

 Sometimes part of a cavity has been filled in one way, and 

 the remaining portion in the other. Occasionally, from 



cavities left after a part of the 

 hollow has been filled horizontally, 

 stalactites of the matter of the 

 agate have descended from above, 



IS 



as in the annexed figure. It 

 desirable always to ascertain how 

 far such flat layers correspond with the present horizon ; 



if the vesicles or hollows are almond-shaped (elon- 

 gated more in one direction than another), how far these 





are constant in the same direction, thus pointing out that 

 in which the molten viscous rock moved. 



Many nodules in rocks, those which have clearly not 

 been formed as gravel or boulders by attrition, aflbrd 

 examples of the aggregation of similar ma tter from a mass, 



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uch as one of day, in which that matter has once hecn 



In this way we have siliceous 



more 



iliised 



calcareous nodules, and those valuable nodules 

 the clay ironstones. These last are fundamentally car- 

 bonates of iron, with a variable addition of the matter of 

 the mud or silt anud which the carbonate of iron has once 



been more generally diffused. 



In many such nodules 



there has been a shrinking from 

 the centre to the sides, causing 

 cracks, that have been va- 



riously filled with mineral mat- 



ter 



as in the subjoined figure 



Occasionally in the cracks so formed, and not quite filled 





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