560 REPORT OF NATIONAL MUSEUM, 1893. 



Au intermediate stage in the process is shown in the small block 

 figured on pi. 8, where oxidation has gone on from all sides until only 

 a nucleal mass of green remains, into which the oxidizing process was 

 extending along the lines of deposition, much as the process of serpen- 

 tinizatiou extends along the curvilinear cracks of an olivine granule. 



Two independent analyses of the green and oxidized portions from 

 this specimen (67825) yielded in the one case 5..j1 per cent, of iron car- 

 bonate (FeCOa) for the green variety, and 4.06 per cent, of the carbon- 

 ate, and 1.73 per cent, of sesquioxide (Fe20,t) for the brown. In the 

 second case 4.27 per cent, of FeCOs in the green variety, and but 1.22 

 per cent, of the same salt in the brown, with 3.53 per cent, of the ses- 

 quioxide. In both cases the total amount of iron calculated as Fe seems 

 a trifle the largest in the oxidized portion (2.65 per cent, in the green 

 as against 3.06 per cent, in the brown). 



The completed stage of oxidation is shown in pi. 15. The original 

 green color is wholly effaced, and the block, when cut across the grain, 

 gives a unique combination of red-brown colors which, together with 

 the original lines of deposition, give an appearance so like that of cer- 

 tain tapestries that I have given it the name of tapestry onyx. 



In a few instances the shades of color are produced by mechanically 

 included impurities, as in pi. 12, from a specimen from San Luis Obispo, 

 California. Visitors to the California pavilion in the mines building 

 during the Exposition at Chicago in 1893 will recall the beautiful and 

 unique pictures in stone there shown. This coloring matter, in its 

 various shades of smoky brown, is due to inclusions of clay parallel to 

 the plane of deposition. It would appear that during the time the 

 stone was being deposited the waters became temporarily charged with 

 silt, which settled in thin films over the uneven, often botryoidal sur- 

 faces already formed, to become entombed in the mass of the stone 

 when the onyx-forming stage was resumed. 



In structure the onjx marbles are invariably holocrystalline, some- 

 times granular, but much more commonly with a fibrous or radiating 

 columnar structure, the fibers or columns being composed of calcite 

 crystals elongated in the direction of their principal axes and standing 

 at approximately right angles to the jdane of deposition, as noted by 

 Sorby* in deposits of similar origin. 



Twin forms so characteristic of the calcite of metamorphosed sedi- 

 mentary deposit, or even the secondary calcite in veins and cavities of 

 eruptive rocks, are quite lacking. Mechanical inclosures of any kmd 

 are almost wholly wanting, as may be inferred from the analyses. The 

 banded aragonite from New Mexico (60631) shows the dark color to be 

 due to included particles of a coal-black color, which give reactions for 

 manganese oxide. 



The characteristic banding or •' grain" of the stone is due to lines of 

 accretion comparable with the lines of growth upon the trunk of a tree, 



* Quarterly Journal of the Geological Society of London, Vol. xxxv, 1879, p. 73. 



