ORIGIIC OF THE CEMEXTIXG MATERIAL 151 



the cement. There is also no doubt that at least some of the calcium is 

 derived from tlie serpentine, for varieties of calcium-bearing serpentine 

 similar to those of analyses B and C are also found in the WMte Creek 

 area, together -with glaucophaue-bearing schists high in calcium, much 

 like the one of analysis D. It is therefore easy to account for the pres- 

 ence of all of the minerals in the cement, and to imagine how they are 

 first dissolved from their original source by the carbonated surface waters 

 under the peculiarl}- advantageous subterranean conditions prevalent in a 

 region of obviously intense alteration; how they then emerge in solution 

 in the waters of the springs on the slopes of the serpentine ridge; and, 

 finally, how, when evaporation has brought the water to a point of satu- 

 ration, the cement is deposited on and between the pebbles and boulders 

 of the stream bed, forming the conglomerate. A combination of mag- 

 nesium and calcium carbonates dolomite ^ and silica is only slightly 

 soluble in water under surface conditions; so that, when once deposited 

 with the associated minerals as a hard cementing matrix: between the 

 pebbles and boulders along the stream bed, the resulting conglomerate is 

 necessarily very resistant to erosion and permits cutting of the con- 

 stituent boulders rather than removal of these rocks from the matrix 

 \\'hen attacked by running water. 



The water as it emerges from the serpentine area is clear, but decidedly 

 hard and mineralized, as may be recognized by the taste, which is, how- 

 ever, not unpleasant. The gradient and rough course of the stream are 

 such that the water is continually churned up on the rocks of the minia- 

 ture rapids, dashed on the adjoining boulders, and sometimes caught in 

 side pools. Under such conditions and in the hot dry summer climate of 

 the region, the evaporation is rapid, and the water, which starts out from 

 the springs along the head of the stream with possibly only a moderate 

 proportion of dissolved mineral matter, very soon reaches a point in its 

 course where the saturation is complete enough to cause deposition. In 

 this connection it must be remembered that, owing to the great evapora- 

 tion during the day, the flow during the latter part of the night and early 

 morning is perceptibly greater and side pools are filled and rocks wet that 

 later on are left unprotected. The diurnal change in temperature and 

 flow are therefore prominent factors in causing the deposition of tlie 

 cement. 



The principal reason why the cementing process is not active in Los 

 Gatos creek below the mouth of White creek is because the more easily 



' A discussion of the properties of this mineral, together with references to a con- 

 siderable number of papers relating to it. will be found on pages 480 to 490 of Dr F. W. 

 Clarke's "Data of geochemistry,'" U. S. Geological Survey. Bulletin no. 380, 1908. 



