Mineral Matters in Sedimentary and Crystalline Pearls. 613 



2. All these three kinds of concretions are formed in consecutive 

 strata round a nucleus, and the strata are not only independent and 

 separable from each other, but so loosely adherent that they can 

 be shelled off like the different layers of an onion, leaving the sub- 

 jacent layers so perfect that their enucleated portions constitute for 

 themselves a perfect pearl, a perfect calculus, or a perfect pisolite 

 concretion. 



3. By the specimens exhibited, it was shown that the outward 

 appearance of a pearl is in no case a reliable criterion of its internal 

 structure, a dull-white sedimentary pearl appearing exactly the same 

 under the microscope as a beautifully brilliant iridescent one ; while 

 a black-coloured pearl may possess in its interior a snow-white pearl, 

 another of the purest water may consist of nothing but a dirty 

 greasy lump of river clay, being in reality merely thinly coated over 

 with iridescent pearl substances. 



4. Thin sections of sedimentary pearls, when viewed with a high 

 power in a good light, have all more or less a granular appearance, 

 in general best marked along the lines of stratification. 



5. All sedimentary pearls have nuclei round which the concentric 

 strata are regularly arranged, the nucleus being sometimes small, 

 sometimes exceedingly large in proportion to the size of the pearl. 

 And, contrary to the opinion of De Filippi,* who asserts that the 

 nucleus is invariably an Entozoon, it may consist of inorganic as 

 well as of organic foreign material a pellet of clay, a particle of 

 sand, a fragment of bone or of iron, a piece of seaweed, or even an 

 entire animal. 



In the second division of his subject, the author demonstrated by 

 specimens that just as a sedimentary pearl closely resembles the sedi- 

 mentary concretions met with in the mineral as well as in the animal 

 world, so in like manner the crystalline form of pearl has its exact 

 counterpart not alone in organic cholesterin gall-stones and carbonate 

 of lime calculi, but in mineral nodules of wavellite rock and balls of 

 iron pyrites. 



As regards the nuclei of crystalline pearls, it was shown that in 

 most instances they are identical with those met with in the sedi- 

 mentary variety of pearls, though sometimes 110 nucleus whatever is 

 to be met with even after prolonged search. Consequently, the author 

 inclines to believe that crystalline pearls in some instances begin (as 

 in the case of natural as well as artificially prepared calculi of 

 the carbonate of lime, as Raineyf showed), by the mere aggregation 

 and coalescence of mineral molecules. 



The author also called attention to Bronn's statement that pearls 

 have often a crystalline nucleus, adducing evidence in favour of the 



* Muller's ' Archiv,' 1856, p. 490. 



t ' The Formation of Bone and Shell Structures,' 1858, pp. 11, 12, and 35. 



