Lawson. | 



Post-Pliocene Diastrophism. 



149 



been depressed over a mile beneath the waters of the Pacific, buried 

 beneath that thickness of strata, and has since, in the most recent 

 geological epoch, been elevated far above tide. It is this differ- 

 ential movement, of a mile or more, of the country to the south- 

 ward of Lake Merced which has caused the deformation and tilting 

 of the Merced' series. 



Discrimination of Orogenic and Epeirogenic Effects. — In the con- 

 sideration of the diastrophism which has affected the San Francisco 

 peninsula we have, then, two displacements to deal with, the oro- 

 genic, or local, and the epeirogenic, or general uplift. Is it possible 

 to discriminate the two effects ? — Certain striking facts suggest an 

 answer to this question in the affirmative. The orogenic uplift pre- 

 ceded the epeirogenic, and there was an interval between the move- 

 ments in which a great denudation was affected. The facts are as 

 follows: The line of demarkation between the Pliocene and the 

 Mesozoic terranes is a nearly straight line coincident with the pro- 

 jection of the axis of the San Andreas Valley to Mussel Rock. 

 This line also separates a country which, in general, has an altitude 

 of less than 700 feet (the Pliocene terrane) from a more elevated 

 tract (the Mesozoic and granite terranes), which culminates in Mon- 

 tara Mountain at 1,940 feet above sea. To the northeast of this 

 line the Pliocene prevails in great volume, as above described. To 

 the southwest the country has now practically no mantle of Plio- 

 cene, although such a mantle certainly once existed, since the Plio- 

 cene strata structurally arch over it. An occasional pebble is all that 

 remains of the Pliocene strata on the higher ridges and along the 

 shore on the lower slopes. Only insignificant patches of these 

 formations may be found between Mussel Rock and the vicinity of 

 Pillar Point, where the Pliocene again appears in force, and con- 

 tinues down the coast for several miles. Such remnants, with the 

 exception of a single pebble, have not as yet been found above an 

 altitude of 720 feet, although the peninsula, as far south as Pillar 

 Point, has been carefully examined. How is it that the Pliocene 

 to the northeast of the line of demarkation has been preserved so 

 intact, while the country to the southwest, above the altitude of 

 say 700 feet, has been completely denuded of the same formations? 

 — An adequate explanation may be found in the hypothesis that the 



