344 PRINCIPLES OF STRATIGRAPHY 



consolidated in this manner and by the formation of conglomerates 

 with a limestone matrix or cement within deposits of the Pleisto- 

 cenic ice sheet, the lime in this case being commonly derived from 

 the solution of limestone fragments included in the deposit. 



The Onyx Marble. A remarkable type of spring deposit is the 

 so-called onyx marble or Mexican onyx so extensively used for 

 decoration and other purposes. (Merrill-35.) This is a compact, 

 highly crystalline and commonly beautifully variegated deposit of 

 calcium carbonate, which occurs interbedded with breccias, or with 

 ordinary porous tufas in many localities. In Yavapai County, Ari- 

 zona, these have been found on the side of ravines in crystalline 

 schists which stand on edge and are penetrated by basic dikes. 

 The deposit occurs interbedded with breccias made of the country 

 rock, and forms irregular, somewhat concentric layers, from a frac- 

 tion of an inch to two or more feet in thickness, and lying essen- 

 tially horizontal. The massive layers are separated from one an- 

 other by porous cellular ones. At Cave Creek, Arizona, large 

 blocks of the onyx up to 4 feet or more in diameter are embedded 

 in tufaceous material. In Mexico, whence the original supply came, 

 this rock is found in masses up to ten feet or more in diameter in a 

 tough, reddish or dark-brown clay, overlying a closely cemented 

 conglomerate. In one instance (at Antigua Salines) it is found in 

 a hard, flint-like country rock, and appears to be a regular vein 

 formation. Similar onyx deposits are found in western Utah. 



In Lower California this onyx marble occurs in the desert region 

 near the Gulf coast. It is found here in layers, slabs and blocks 

 20 inches to 3 or more feet thick, interstratified with tufaceous beds 

 and ancient lake bed material, and shows a most surprising variety 

 of delicate colors and tints. Elsewhere in lower California on the 

 steep slopes of Tule Arroyo, a deep canyon in dark mica schist and 

 blue-gray silicified limestones and quartzites standing on end, simi- 

 lar deposits of these onyx marbles were made by former springs. 

 At first a cellular travertine was formed cementing the angular 

 fragments of the older rocks, with which the slopes were strewn, into 

 a coarse breccia. After several feet of such deposits, the compact 

 onyx marble succeeded, probably through a change in the constitu- 

 tion of the spring or the conditions of deposition. "In time the 

 onyx-forming action ceased — and for a period no calcareous deposi- 

 tion whatever took place, the slopes becoming once more covered 

 with angular particles of the older rocks from higher up, these in 

 their turn becoming cemented into breccias when the springs re- 

 sumed their work. In this way were built up the alternating layers 

 of breccia, tufa and onyx, until finally all deposition practically 



