GEOLOGY. 293 



faith of the mineralogist ; but to those who have seen, as I have clone, 

 the contents of fluid cavities in crystals solidifying and even crvstal- 



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lizing on the face of cleavage, -while another portion of the con- 

 tents of the cavity escaped in gas to those who have seen in 

 topaz cavities numbers of regularly formed crystals, some of which, 

 after being fused by heat, instantly re-crystallized the conclusion I 

 have drawn will be stripped of much of its apparent extravagance. 

 In examining a number of diamonds in the Museum of the East India 



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Company, and about forty or fifty in the possession of Messrs. Hunt & 

 Roskill, I found many containing large and irregular cavities of the 

 most fantastic shapes, and all of them surrounded with irregular 

 patches of polarized light, of high tint, produced undoubtedly, by a 

 pressure from within the cavities, and modified by their form. Among 

 these specimens I found one or two black diamonds, not black 

 from a dark coloring matter, like that in smoky quartz, but black from 

 the immense number of cavities which they contain. Tavernier has 

 described a large and curious diamond which throws some light on the 

 subject of this notice. It contained, in its very centre, a large black 

 cavity. The diamond merchants refused to purchase it. At last a 

 Dutchman bought it, and, by cutting it in two, obtained two very fine 

 diamonds. The black cavity through which he cut was found to con- 

 tain eight or nine carats of what Tavernier calls black vegetable mud ! 

 Mr. Tennant, the celebrated geologist, stated that at the last meeting 

 of the British Association, Dr. Beke read a paper " On the Diamond 

 Slab supposed to have been cut from Koh-i-noor." He stated : "At 

 the capture of Coochan there was found among the jewels of the 

 harem of Reeza Kooli Khan, the chief of that place, a large diamond 

 slab, supposed to have been cut from one side of the Koh-i-noor, the 

 great Indian diamond now in the possession of Her Majesty. It 

 weighed about 130 carats, showed the marks of cutting on the flat and 

 largest side, and appeared to correspond in size with the Koh-i-noor." 

 Prof. Tennant was induced to record his opinion of the probability of 

 this being correct. He had made models in fluor spar, and afterwards 

 broken them, and obtained specimens which would correspond in 

 cleavage, weight, and size with the Koh-i-noor. By this means he was 

 enabled to include the piece described by Dr. Beke, and probably the 

 large Russian diamond, as forming altogether but portions of one large 

 diamond. The diamond belongs to the tesselar crystalline system ; it 

 yields readily to cleavage in four directions, parallel to the planes 

 of the regular octahedron. Two of the largest planes of the Koh-i- 

 noor, when exhibited in the Crystal Palace, were cleavage planes 

 one of them had not been polished. This proved the specimen to be 

 not a third of the weight of the original crystal, which he believed to 

 have been a rhombic dodecahedron ; and if slightly elongated, which 

 is a common form of the diamond, would agree with Tavernier's 

 description of it, bearing some resemblance to an egg. Referring to 

 the diamonds procured in the Brazils, he related a fact which, he said, 

 was told to him by a gentleman from Brazil. A slave in that country 

 was one day wading in a river in search of the precious gems to be 



