THE ONYX MARBLES. 561 



each layer represeiitiug successive surfaces over and ujion which the 

 lime-holding solutions have deposited new materials. In some instances 

 the successive layers vary more or less in character of crystallization 

 and color, due to a slight change in contents of organic matter or metal- 

 lic oxides, or physical conditions, whereby the material, is rendered 

 more or less opaque. The characteristic feature which above all others 

 adds beauty to the stone is its trauslucency, which is a purely physical 

 quality. As a rule the crystallization, in sound blocks, continues unin- 

 terruptedly upward through the successive layers for a distance of 

 several millimeters, so that there is no tendency toward separation 

 along these layers until a point is reached where, owing to impurities 

 in the water, or it may be a temporary cessation of deposition, crystal- 

 lization ceased. On beginning once more, such lines not infrequently 

 form lines of weakness. A not uncommon structure is that shown in 

 lig. 1 of pi. 9. Crystallization starts from a series of points on a 

 preexisting surface and progresses upward and outward forming 

 a series of inverted cones. This structure is evident only on close 

 inspection and in slabs suflticiently thin to be translucent. 



Asa natural consequence of its mode of deposition the surface struc- 

 ture is usually botryoidal. Cut across the plane of deposition the 

 structure is then as shown in flg. 1 of pi. 1. Cut at right angles to 

 this, the structure, owing to the wavy, botryoidal nature of the original 

 surfaces, is often wonderfully beautiful and always interesting. The 

 colors continually appear and reappear in varying degrees of intensity 

 accordingly as they lie upon the immediate surface or are subdued by 

 intervening layers of colorless material. One sees in fact not merely 

 the colors which lie upon the surface, but those beneath as well, sub- 

 dued, enhanced, enriched it may be, by those which overlie or lie 

 beneath. It is in this characteristic that lies the chief claim for beauty, 

 and its entire separation from marbles of the common, sedimentary 

 type. The figures given on the pis. 14, 17 and 18 will serve to show, 

 so far as is possible by photograph, the varying structure described. 



The cave marbles are as a rule much less translucent than the trav- 

 ertines, coarser in crystallization, and hence more liable to fracture. 

 They are, moreover, less homogeneous, containing many cavities and 

 interspaces which have never been filled. In the columnar forms a 

 pronounced zonal structure is common, as shown in the cross and longi- 

 tudinal sections in figs. 3 and 4 on pi. 13. In the more massive forms 

 we find the bandings as in the travertines, but without the delicate 

 crystallization. 



LOCALITIES: DOMESTIC AND FOREIGN. 



Arizona. — Several deposits of onyx have within a few years been 



located in Arizona, though so far as at present known to the writer, but 



two are of such extent as to be of any commercial value. These two 



are both in Yavapai County, and possess many characters in common. 



H. Mis. 184, pt. 2 36 



