566 H. W. FAIRBANKS 
have decidedly the character of fresh-water lake beds, and there 
is no reason to believe that they were ever connected with the 
Merced beds. 
PLEISTOCENE. 
Under this designation are included the terrace formations, 
blown sand, stream gravels, and alluvial bottoms. These were 
not all formed at once, but represent a complicated history. 
The terrace formations belong to the oldest division. The most 
extensive area included under this head embraces a great stretch 
of sandy mesas and hills between the mouth of the Arroyo- 
Grande Creek and the Santa Maria River. 
The coast, almost everywhere more or less distinctly ter- 
raced, forms an elevation of about ten feet above mean tide up to 
750 feet. In addition terrace-like effects were noted upon the 
southern slope of the Santa Lucia range at elevations of 1000, 
1400, and 1700 feet, aneroid measurements. In many places 
along the upper Salinas there are beautiful examples of river 
terraces cut upon the Paso Robles formation. 
IGNEOUS ROCKS. 
The igneous rocks of the San Luis Obispo region form one 
of the most interesting features of that field. Intrusives and 
surface flows are numerous, especially the former. They are so 
abundant in the Golden Gate series as in many places to form 
fully one-third of the surface exposures. The extensive sheets of 
diabase and peridotite appeared during the Cretaceous, and a 
great variety, though of much lesser extent, during the Miocene. 
The age of the andesite and dacite granophyre forming the 
buttes reaching from San Luis Obispo to Morro Bay cannot be 
determined with certainty. It certainly antedates the peridotites 
which appeared at the close of the Knoxville and may be of pre- 
Cretaceous age. 
The igneous rocks, all taken together, show a great range in 
chemical and mineralogical composition, with examples of rare 
and interesting types. They have not yet been thoroughly 
