1 ,AW ON. I Post-Pliocene Diastrophism. [43 



ocene rocks here referred to will be designated the Merced series 

 from Lake Merced, which lies in a structural or synclinal depression 

 of the Pliocene terrane, to the south of the city. The base of the 

 Merced series is observable at Mussel Rock. The basal bed is a 

 stratum of partially carbonized forest material, from which fragments 

 of little altered wood, bark, branches, matted leaves, and pine cones 

 may be gathered ad libitum. The cones, according to Prof. E. L. 

 Greene, who kindly examined them for me, are those of Pinus insig- 

 nis (Monterey pine), a tree which at the present time grows only at 

 Monterey. The forest bed rests directly on a seemingly even sur- 

 face of volcanic rocks (pyroclastic and massive) which are of Meso- 

 zoic age. This surface slopes now to the north at an angle of 15 . 

 It is a surface of truncation by erosion and doubtless represents a 

 pre-Pliocene base-leveled plain. 



From this basement at Mussel Rock the strata of the Merced 

 series are well exposed in ascending sequence to Lake Merced. 

 The edges of the strata form the sea-cliff for the entire distance. 

 The sea-cliff is in active recession, so that fresh exposures of the 

 rocks are afforded throughout the section. The strike is for the 

 most part more or less transverse to the shore, and the latter is a 

 simple, nearly straight line. The rocks are tilted, generally at high 

 angles, and have a monoclinal structure for the entire length of the 

 section. The sharpness of the strata and the comparative constancy 

 of both dip and strike favor precise observation. There is no repe- 

 tition of strata, and fault structure is represented only by very minor 

 dislocations. In a word, the section is ideally simple, and is emi- 

 nently susceptible of approximate!}' accurate measurement for the 

 thickness of the series. There is but one drawback, and that con- 

 sists of the two land slides which scar the face of the cliff. These, 

 however, do not practically affect the results set forth in the sequel, 

 since one occurs where the strike is locally parallel to the line of 

 section, and is therefore not considered in the calculation, while the 

 other, near Mussel Rock, although not showing the strata in the 

 sea-cliff shows them in the cliff behind the slide. The cliff at its 

 highest is about 720 feet above the shore, and at several places the 

 strata may be seen extending from the shore to the top of the cliff 

 with uniform dip. The rocks are chiefly soft sandstones of gray 



