158 INCBUSTATIONS, ETC. 



tinated together. Occasionally each pisolite encloses in its 

 centre a grain of foreign substance ; but this is by no means 

 a necessary condition, and the original nucleus may be a 

 particle of the same substance in which it is enclosed. The 

 oolitic structure, particularly of the coarser kind, strongly 

 resembles the pisolitic, and is considered to be owing to a 

 similar cause. 



ROLLED PEBBLES. Running waters have the effect of 

 rounding those materials which they wash out of the beds 

 by the attrition and abrasion occasioned by such transport. 

 A familiar illustration is furnished by the beach formed at 

 the base of chalk cliffs, where none of the flints present sharp 

 edges, being all worn smooth by the action of the waves. 



NODULES. Substances which have consolidated in the 

 midst of semi-fluid matters often owe to the resistance thus 

 opposed to them a reniform or kidney-like shape, such 

 objects being smooth externally, when the substance of 

 which they are composed is little susceptible of crystal- 

 lisation, and furnished with crystals when it is. The former 

 case is instanced by nodules of silex ; the latter, by various 

 substances of more crystalline character, among others, the 

 masses of iron pyrites discovered in the same deposits. 



G-EODES. Occasionally nodules of silex, and other sub- 

 stances, are hollow in the interior, and they are then termed 

 geodes. The nodules of quartz, which present this form, are 

 usually lined with crystals of the same substance; they 

 occur abundantly in the counties of Somerset and Devon, 

 where they bear the name of potato-stones. 



INCRUSTATIONS AS DISTINGUISHED FROM PETRIFAC- 

 TIONS. Waters which flow over limestone rocks have the 

 power of dissolving a portion of the lime, and on reaching 

 the colder temperature of the atmosphere, they deposit 

 it again in a solid state ; a familiar example of this opera- 

 tion being furnished by the fur, as it is termed, of the tea- 

 kettle, which is formed by the water which has boiled, 

 having deposited, on cooling, a portion of the lime which it 

 previously held in solution.* Certain springs in this country, 

 as well as on the Continent, are famed for this property.; 

 among the former, those of Knaresborough and Matlock are 



* The carbonic acid of the water held the lime in solution ; boiling expels 

 the gas, and causes the lime to fall down as a sediment. 



