NEOCENE STRATIGRAPHY. 315 



Still shows, but in most cases it is invisible macroscopic- 

 ally. 



At the top of the section along Seven-Mile Beach, the 

 sandy strata become quite soft — there are found to be no 

 hard strata. As we rise in the series the strata become 

 more and more unconsolidated and sandy, the upper layers 

 consisting almost entirely of yellow and orange sand. 



Fine gravel occurs in some abundance, and there is a 

 little coarse gravel, but in no case is it consolidated into a 

 conglomerate. 



The most noticeable layer in this upper series is a white, 

 chalky layer, which Dr. Lawson* considers a volcanic 

 ash. On the beach it appears as a bed having a uniform 

 thickness of about one foot, interbedded with the other 

 strata. At one point on the northeast slope of the bluffs, 

 near the head of Lake Merced, it has a thickness of six 

 feet. The Spring Valley Water Works Company mine 

 it here for " chalk." Under the microscope it shows no 

 crystalline structure, nor could any diatoms or foraminifera 

 be made out; a chemical analysis would probably throw 

 much light on its composition and origin. 



Stratigraphy. — The best section obtained was that 

 along Seven-Mile Beach. This section gives probably a 

 nearly complete series at the top. The bottom of the 

 section, however, is about 670 yards north of the contact 

 with the igneous rocks of Mussel Rock, so that the sec- 

 tion is not complete; but the dip at this point becomes 

 low, and there is abundant evidence of faulting, so that 

 it is not thought that more than a few hundred feet are 

 omitted. 



A study of the contact between the Merced series and 

 the igneous rocks from Mussel Rock southeastward shows, 

 however, that the bottom of the series has not been ex- 



* Univ. of Cal., Bull. Dept. GeoL, vol. i,p. 144. 



