548 REPORT OF NATIONAL MUSEUM, 1{^93. 



thus deposited was a dull-colored cellular travertiue which cemented 

 together the augular fragments of older rocks with which the slopes 

 were strewn, giving rise to a coarse conglomerate, or breccia. After 

 this had gone on for some time and the travertine layer had grown, it 

 may be, to several feet in thickness, the conditions changed as I 

 have attempted to outline above, and the deposit took the form of the 

 compact and beautifully veined and tinted stone to which the name 

 onyx is commonly applied. In time the onyx-forming action ceased in 

 its turn, and for a period no calcareous deposition whatever took 

 place, the slopes becoming once more covered with angular particles 

 of older rocks from higher up, these in their turn becoming cemented 

 into breccias when the springs resumed their work. In this way were 

 built up the alternating layers of breccia, tufa, and onyx, until finally 

 all deposition practically ceased, and spasmodic but fiercely rushing 

 streams cut the arroyo to its present depth, exposing in either wall the 

 irregularly alternating beds described. In the bottom of the caFion 

 still exist two diminutive springs, each building up in its feeble way 

 small beds of tufaceous material. The water still flowing is so highly 

 carbonated as to bubble like a glass of Vichy fresh from the fountain, 

 and leaves, when drank immediately, a pleasant prickly sensation in 

 the mouth and throat 



In discussing the origin of the onyx marbles, it is perhaps but fair 

 to question the possibility of their having been originally deposited as 

 tufaceous materials, and subsequently compacted and crystallized by 

 pressure, heat, and percolating solutions, or other of the ordinary agents 

 of metaniorphism. This can be best answered by pointing out the 

 alternating character of tufaceous and compact layers. It is ditticult 

 to conceive of conditions such as should have metamorphosed any one 

 bed without aft'ecting, in the least, those either above or below, or both. 

 The wavy, undulating lines of deposition, comparable with the rings of 

 growth upon a tree, are well defined and unbroken, and though ditter- 

 ing frequently from one another in color and crystallization preserve 

 their parallelism and individuality throughout. (iSee pis. 1 and 3.) 

 There are apparently good reasons for supposing the material to have 

 been deposited as we now find it, the crystallization being contempo- 

 raneous with deposition, as is the case with the stalagiuitic material in 

 caves. * The wonderful variations in color, even in the same block or 

 slab, are, however, in part due to changes subsequent to their deposi- 

 tion, and it may be well to dwell upon this branch of the subject in 

 considerable detail. Few rocks possess so wide a range of colors or 

 shades of the same color. Pure white, opaque, milk or chalk white to 

 almost colorless, gray, brown in hues from light ochre to deep mahogany, 

 buff, amber, ochre yellow, pink, red, and green are all common; the 



* These latter deposits tlo, however, in some cases undergo a recrystallization 

 whereby the whole internal structure is luodiiied without change of external form, 

 as 1 have mentioned elsewhere. 



