THE ONYX MARBLES. 547 



the dei)Osit to assume a crystalliue structure, we miglit at first thought 

 conclude these to be cold rather thau hot water deposits. Bearing in 

 mind, however, that they bear evidence of comparatively rapid growth, 

 such as would indicate deposition from waters of a high degree of sat- 

 uration, it would seem more probable that at the time of issuing the 

 waters were comparatively hot, and perhaps under conditions of con- 

 siderable pressiire as well as saturation.* The tendency of such to 

 immediately lose their excess of carbonic acid and deposit a light 

 tufaceous travertine or sinter, as upon the immediate surface at the 

 Mammoth Hot Springs, would be checked provided the discharge took 

 place in pools of quiet water. We know that deposits sufficiently com- 

 pact to receive a polish are sometimes formed in steam boilers, where, 

 however, more than ordinary degree of saturation prevails and under 

 unnatural conditions of pressure, t The onyx deposits being, how- 

 ever, purely superficial, no such conditions of pressure could exist, and we 

 must apparently fall back upon such conditions as should retard the 

 loss of carbonic acid and thus cause the deposit to take place more 

 slowly. Such conditions, we may fairly assume, would exist at the 

 bottom of pools of water, and it is under such conditions, in the 

 writer's opinion, based upon observation as well as on theoretical 

 grounds, that the onyx marbles have been formed. To account, then, 

 for the alternating tufaceous and compact character of the beds which 

 everywhere exists, we have to make only the natural assumption that 

 the temperature of the water and its degree of saturation periodically 

 varied, the variation being accompanied perhaps by a difference in 

 volume or place of discharge, whereby the water hitherto accumulating 

 in pools, ran off almost immediately, permitting a ra})id loss of carbonic 

 acid and an equal rapid diminution in temperature. This intermittent 

 character of the deposition, and in fact the general history of onyx 

 formation is, so far as my own experience goes, best shown in a region 

 rather difficult of access known locally as the Tule Arroyo, a deep cation 

 or ravine on the peninsula of Lower California, some 150 miles south from 

 San Diego, and 15 to 20 miles from the gulf coast. The country rocks 

 here are nearly black mica schists and blue gray silicifled limestones 

 and quartzites standing nearly vertical with a strike some 20^* west of 

 north, the whole being cut by the ravine, or arroyo, as it is called by 

 the Mexicans. On the steep slopes of the hills on either side, and 

 before the ravine had assumed anything like its present depth, springs 

 have from time to time gushed out and deposited their calcareous load 

 upon the surface over which they tlowed. As a rule the first material 



*T]ie fact that nearly every deijosit of tliis nature of which I have thus far found 

 trace is iu a region of comparatively recent volcanic activity increases the probabil- 

 ity of their being hot-spring deposits. 



tTbe writer has in his possession such a crust some .5 c. m. in thickness, taken from 

 the boiler of an ocean steamer plying between New York and Portland, Me. In this 

 case, however, the mineral is anhydrite (anhydrous sulphate of lime) rather than a 

 carbonate. 



