NEOCENE STRATIGRAPHY. 313 



Mile Beach becoming over 700 feet high. It is doubtful 

 if there is anywhere in America a Tertiary formation 

 presenting so many advantages and attractions for study. 

 That it is intimately concerned in the structure of the 

 mountain, that it is cut and interbedded by igneous erup- 

 tions, and that during the deposition of its nearly a mile 

 of sediment a gradual but marked change in the fauna 

 can be traced, increases the interest it must have for the 

 student. 



Petrograj[)hy. — The Merced series of the Santa Cruz 

 Mountains is composed of a great thickness of partly 

 consolidated sands, clays, argillaceous sands and hard, 

 fine conglomerates. On the bay side of the range the 

 strata have felt the mountain-making forces more, and 

 are usually harder and, as shown by the distortion of the 

 fossils, more crushed. Local metamorphism and minor 

 variations will be mentioned later. The most abundant 

 and characteristic rock is a dark drab or slate colored 

 argillaceous sand, breaking into small fragments of about 

 half an inch cube, which in many places are bright red 

 on the joint faces. In places it shows a yellow ochre-like 

 deposit. It varies in hardness from that which crushes 

 easily in the hand to tough and more argillaceous varieties 

 which are like a hard clay. Locally it sometimes forms 

 very hard nodules or layers, generally due to the lime 

 from inclosed shells, many of the shells collected being 

 obtained by splitting open these nodules. In places this 

 sandstone is thin-bedded, but it is more apt not to show 

 many bedding planes in a thickness of 100-200 feet. 

 This rock makes up the most of the long hill at Point 

 Pillar, and likewise much of the bluffs to the south and 

 on Seven-Mile Beach. 



The next most abundant rock is a yellow or buff colored 

 sand, generally quite soft; sometimes it weathers in the 

 bluffs until it seems to be filled with great pot-holes. 



