GEOLOGY OF CARRIZO MOUNTAIN, CALIFORNIA 345 



In this region the accumulated effusive materials are best dis- 

 played in maximum thickness in the ridge that has been mentioned 

 between Barrett and Deguynos canyons. They must be more than 

 500 feet thick here. In Alverson Canyon their mass is much less. 



Miocene conglomerates. — In the lower part of Alverson Canyon 

 (Fig. 4) a heavy conglomerate bed 120 to 130 feet thick overlies a 

 series of tuffaceous strata. This bed is composed of coarse material 

 at the base but becomes finer toward the top. It is only moderately 

 hard and along its upper margin is an abundantly fossiliferous horizon. 

 Splendid coral heads are imbedded in these sandstones, and more 

 delicate forms are found at the base of the superjacent sandy shales. 

 These corals with the molluscan remains that accompany them, all 

 of which await detailed examination and determination, prove the 

 age of the inclosing rocks to be upper Miocene. 



On the north slope of Carrizo Mountain, about the head of the 

 eastermost arroyos which are tributary to Garnet Canyon, another 

 series of fragments of a well-developed basal conglomerate are 

 encountered. They extend well up the slopes of the older meta- 

 morphic rocks which form the axis of the mountain and dip away 

 from it toward the north or northeast at the rate of 20 or 30 degrees. 

 Being more resistant to weathering agencies than the soft overlying 

 shales, these have been stripped from the sandstones at many points 

 so that the old Miocene beach (Fig. 5), its sands indurated and its 

 teeming life preserved only in fossil form, but yet exhibiting much 

 the aspect and much the same relations which existed at the time of 

 its deposition, is revealed for the modern student's inspection. These 

 basal sands are not always found where their horizon is exposed. 

 In many places the fine clays that were spread out over the sandstones 

 were deposited directly upon the metamorphic rocks that form the 

 core of the mountain and must at one time have formed the bottom 

 and shores of the Miocene sea. The simplest interpretation of this 

 relation is to suppose that before that change of conditions was com- 

 plete which substituted muddy brackish water with oyster colonies 

 for clear sea water and marine hfe, the sands of the earlier beach 

 had been swept away, so that there is unconformity, without dis- 

 cordance, or at least without marked discordance in dips, and with- 

 out a great time interval between the deposition of the sands and 



