pS SOUTHERN CALIFORNIA ACADEMY OF SCIENCES. 



( x\ndrevv C. Lawson, in Bulletin of the Department of Ge- 

 ology, University of California, Vol. I., No. 4, p. 158.) 



Professor Lawson also notes the presence of pliocene de- 

 posits of one mile in thickness, lying- south from San Francisco. 



The evidences of physiographical changes near the shore 

 line are abundant and well marked. 



The question of how and when the benches or shore terraces 

 were formed has caused much discussion among geologists, 

 among whom they are generally considered as water-formed de- 

 posits, and entirely the result of changes in sea level. 



Professor George Davidson, for many years the able Super- 

 intendent of the United States Coast and Geodetic Surveys, who 

 had unrivalled opportunities for the detailed study of our coast 

 line, is of the opinon that the terraces resulted from the action of 

 ice sheets, or an ice belt contiguous to the continental shores, 

 which he claims skirted our shore, and bv its continuous move- 

 ment planed down the irregularities of up-tilted and contorted 

 surfaces of rock of varied character. He says "That some few 

 of the smaller ones which are composed of gravel, etc., were made 

 by the action of water, and may mark ancient sea levels, may be 

 admitted ; but those that exhibit on an extended scale level pla- 

 teaus of rock, which have every degree of inclination, and an in- 

 finite variety of texture, cannot have been so wrought. See 

 Plate I. 



Other forces more powerful and more uniform and constant 

 in action than water, shaped these flat-topped rocky benches or 

 plateaus." 



An examination of some of these plateaus will show that the 

 later deposits of gravel, sand, silt, etc., lie unconformably on a 

 surface of rock which appears to have been absolutely planed 

 off and the different degrees of hardness of the stratification have 

 no apparent influence upon the mechanical forces at work." 



It is probable that both of these theories are correct, in part, 

 and that in some localities, at least, the ice planed down the first 

 or oldest plateau, upon which were subsequently deposited the 

 more recent formations, which, by the elevations of different ep- 

 ochs, formed the raised sea beaches or plateaus. 



The islands forming the southerly boundary of the Santa 

 Barbara Channel, present many interesting features illustrative 

 of the changes in the topography or physiography of the region. 



These islands were formed either by an overflow of lava 

 from some crater on tne mainland, the locality of which is un- 

 known, or, by the opening of a fissure by the pressure of molten 

 lava beneath the surface, which released it and allowed it to flow 

 out and fill up the valley's and other depressions on the surface, 



Of these two theories the latter seems to be the most plau- 

 sible. 



The earlier flows of lava were, after cooling, broken up into 



