GEOl.OGICAL STRUCTURE — WESTERN RANGE. 503 



canyon walls are eroded into castellated forms that recall the buttes at 

 Green river, Wyoming, familiar to travellers on the Union Pacific railroad. 

 Opposite Rosario the beddinoj planes have a dip of 15 degrees to the 

 northeastward, while the surface of the mesa is quite horizontal, and 

 from the pebbles and recent shells on its surface evidently represents a 

 higher level of the ocean waters, which have baselevelled it at about 1,000 

 feet above present sealevel. For a few miles north of the mouth of the 

 Rosario canyon the bluffs come close to the present coastline and then 

 gradually retreat, until opposite San Quentin they are about eight miles 

 inland. The immediate shore is first a terrace about 200 feet above sea- 

 level, then at the mouth of the Sorrocco valley a triangular shaj^ed 

 Quaternar}?- delta hardly 50 feet above sealevel, covered with rolled 

 pebbles and recent marine shells. The older beds forming the mesa 

 region in this latitude, though not markedly different from those between 

 Bluff and Canoas points, contain a larger proportion of conglomerate 

 material and several fossiliferous beds of recent looking shells, among 

 which were recognized Mjlilus californianus and a fragment of a Pecten, 

 like P. cerrosensis, which Dr Dall regards as indicating a probable Miocene 

 age. These are the beds seen by Gabb on his trip and called by him 

 " mesa sandstones." No evidence of unconformity between these and 

 the Tejon beds was observed, and it seems probable that they may con- 

 stitute the highest part of the mesa at Bluff point, but this was not 

 determined by fossil evidence. 



Northward from Sorrocco river the bluffs of the mesa formation retreat 

 gradually from the ocean, and at San Quentin are separated from it by 

 the sand}' plains of Santa ]\Iaria, about 8 miles wide and but a few feet 

 above sealevel, which are the northern continuation of the depression of 

 the bay of San Quentin. The immediate coastline at San Quentin is 

 formed by a group of six conical hills of basalt, from 400 to 800 feet 

 high, which, judging from the uneroded character of the lava flows which 

 have issued from their flanks, must be of very recent eruj^tion. One of 

 these flows extending southward about seven miles forms the low, narrow 

 tongue of land known as cape San Quentin. It is evidently the superior 

 resistance of these hard lavas that has thus far protected the plains of 

 Santa Maria from the encroachments of the sea. 



M^estern Raaye. — In the present topography the western range is very 

 ill defined, and consists of a number of irregular ridges and isolated 

 mountain masses 15 to 20 miles from the coast, the highest summits of 

 which are probably less than 4,000 feet above sealevel. Between the 

 peaks are broad transverse valleys and flat topped ridges whose higher 

 summits have the same general level with those of the higher plateaus 

 of the mesa region, that is, about 2,000 feet. Rounded pebbles and an 



