Lawson. I 



Posl-Pliocene Diastrophism. 



119 



istic and fairly persistent feature of the mesa. And not only is it 

 found on the upper surface of the old delta plain, but also generally 

 strewn over the different stages of the terraced canons and valleys, 

 which have been cut down into it. The lower portion is some- 

 times wanting and the gravel formation rests, as at Point Loma 

 and La Jolla, upon older strata, to which Fairbanks has recently 

 called attention, showing them to be of Cretaceous age.* 



Age. — The Pliocene age of the lower portion of the delta is 

 established not only by a comparison of the formations with the 

 characteristic Miocene strata of neighboring portions of the coast, 

 but also by palaeontological evidence. Several years ago Dall exam- 

 ined a collection representing sixty-nine species from an excavation 

 at San Diego, and pronounced them to be of Pliocene age.f The 

 formation from which these fossils came appears to be the same as 

 that which underlies the gravels of the greater part of the mesa. 



Post-Pliocene marine fossils are also reported by Dall % from the 

 rear of the mesa near San Luis Rey, at an altitude of 600 feet, and 

 twelve miles distant from the shore of the sea. It is not known, 

 however, whether these fossils occur beneath the gravels of the orig- 

 inal delta plain, or whether they are found in the formations of one 

 of the earlier terraces which have been cut into the delta. § 



Uplift. — The evolution of the greater portion of this delta in Pli- 

 ocene time was probably a complex process in detail, and no attempt 

 will here be made to enter upon its elucidation. That would require 

 a prolonged study in the field with good maps. But without dis- 

 cussing the conditions which affected the development of the marine 



* Am. Jour. Sci., Vol. XLV., June, 1893, p. 473. 



t Proceedings of the California Academy of Sciences, April 7, 1874. 



% Proceedings of the U. S. National Museum, Vol. I, 1878, p. 3. 



'i Dall has recently, in his work on the Neocene (Bull. 84, U. S. G. S., PI. 

 II), mapped a strip of Miocene along the front of San Diego County. The only 

 warrant for this seems to be an opinion of Conrad's as to the Miocene age of 

 some fossils in a block of sandstone, from an unknown locality, which was 

 handed to Blake in 1853, when he was in the vicinity of San Diego. The pres- 

 ent writer saw no formations resembling the characteristic white shale of the 

 Miocene of this coast, and he doubts the presence of the Miocene along the 

 coastal slope of San Diego County. The presence or absence of the Miocene 

 does not, of course, in any way affect the evidence of the Pleistocene uplift dis- 

 cussed in this paper. 



