GEOLOGY OR THE SOUTHERN COAST RANGES 573 
miles the center of this syncline has been broken through by 
bodies of olivine diabase. They appear as dikes or bunches 
which as exposed in places have lifted up and inclosed large 
masses of the shale. | 
On Old Creek there are very large sheets of the diabase, from 
the upper surface of which the Monterey shales have been nearly 
removed, as they occur only in patches here and there. These 
igneous masses in many respects resemble laccolites, only that 
instead of having arched the strata in dome form they appear as 
saucer-shaped sheets issuing from lines of fracture along centers 
of syneclines. 
GEOLOGICAL HISTORY. 
The geological history of this region is exceedingly compli- 
cated, and in the short review following only the merest outline 
can be given. The earliest event of which we have any record 
in the Coast Ranges was the invasion of the crust by great masses 
of molten granite, metamorphosing the existing sediments to 
marbles and schists. The date of this convulsion is unknown 
save that it must have long antedated the Jurassic. With the 
beginning of the deposition of the Golden Gate series the great 
area of crystalline rocks which had for so long a time formed a 
land mass in the region of the present Coast Ranges began to 
subside and continued to do so with oscillatory movements until 
its close. Although this formation is so widespread it is essen- 
tially a near shore or shallow water formation as it consists so 
largely of sandstone. The conditions at times changed and the 
sea bottom instead of being subjected to sedimentation from the 
land was covered with the siliceous skeletons of radiolaria which 
it is believed form the main portion of the lenticular jasper beds. 
With the upheaval of the series, and in some cases possibly 
earlier, appeared dikes and sheets of basic igneous rocks which 
everywhere so abundantly characterize it. After erosion had 
planed off the surface flows of basalt took place. 
The period of the intrusion of the more acid plugs forming 
the line of buttes between San Luis Obispo and Morro Bay is 
