614 Mineral Matters in Sedimentary and Crystalline Pearls. 



view that the so-called crystalline nucleus is in many cases but the 

 centre, and therefore really an integral part of the pearl's crystal- 

 line structure ; while, again, in other instances, the appearance is due 

 to the section having been carried a little on one side of the pearl's 

 centre, and a consequent cutting across of some of the hexagonal 

 basaltic-like prisms, producing in the centre of the pearl a tesselated 

 crystalline appearance. 



Bronn farther asserts that in the centre of pearls are occasionally 

 found cells containing calcspar, which has arrived there by a process 

 of infiltration from without. This appearance, the author thought, 

 could be better explained on other grounds, and he exhibited the 

 microscopic section of an oxalate of lime human urinary calculus with 

 a crystalline centre, surrounded by a sedimentary zone exactly as is 

 seen in some pearls, but where no idea of infiltration could possibly be 

 entertained. 



A good section of a crystalline pearl, while showing basaltic-like 

 prisms radiating from the centre to the circumference, equally well 

 shows that it, too, like the sedimentary variety, is formed in concentric 

 layers of stratification. In this respect it not only bears a marked 

 similarity to the carbonate of lime concretions formed artificially in 

 gum-water, but an equal analogy to the sulphate of baryta stalactites 

 found in the caves of Derbyshire, where, of course, it is impossible to 

 attribute the structural arrangement of their mineral matters to the 

 influence of vital energy. 



Microscopic sections of crystalline pearls not alone convey the 

 idea that the prisms branch and interlace with each other, but that 

 they are in some instances of a fusiform shape, like the so-called 

 fusiform cells Carpenter described in shell structures; but these 

 appearances, as the author demonstrated, are simply due to the 

 section having cut the prisms across at different angles. Moreover, 

 the prisms are striated, but this striation, as well as the feathery 

 frond-like appearance some of them present, does not, he thinks, 

 exist in the mineral matter, but in the animal membrane which 

 surrounds each individual prism, and which, as he previously 

 showed, is an integral part of the pearl, amounting to 5'94 per cent, 

 of its total weight.* The feather-like appearance, he fancies, is 

 merely due to a wrinkling of the dried animal matter. It was further 

 pointed out that each individual prism is made up of a number of 

 brick-like segments. Some with a high power appearing striated (6), 

 others granular (a) with knob-like bodies at their corners, and tube- 

 like lines near their layers of stratification. 



The discussion of the arrangement of the animal matter is deferred 

 until after the mineral structure of hybrid, so-called cocoa-nut, and 

 fossil pearls has been considered. 



* " The Chemical Composition of Pearls," 'Koy. Soc. Proc.,' vol. 43, p. 461. 



