1917] Clark: Geology and Ore Deposits of the Leona Rhyolite 365 



of the region which is discussed in the text of the San Francisco Folio. 

 The pebbles of the conglomerate are mostly chert, shale, and sandstone, 

 but along with these are numerous boulders of granite and grano- 

 diorite. It seems, therefore, that there must have been marked dia- 

 strophic movements at the close of the Knoxville epoch, but these 

 movements were not of such a nature as to raise the Knoxville sedi- 

 ments above sea-level. 



After erosion which locally removed the Shasta-Chico rocks, the 

 Leona rhyolite was extruded on the surface as a lava flow. From its 

 present disposition it appears that the flow probably followed an ero- 

 sional valley. The valley must have roughly followed the contact of 

 the Knoxville and the Franciscan. Since the extrusion of the rhyolite 

 there have been several crustal movements. 5 These movements have 

 continued up to recent geologic time, as is shown by the various alluvial 

 deposits, and also by the present streams that in many instances show 

 recent readjustment. The most recent movement has been faulting 

 along a line generally parallel to the Berkeley Hills. 



DISTRIBUTION OF THE RHYOLITE 



The rhyolite forms a narrow discontinuous belt that extends from 

 Hamilton gulch at Berkeley to a point about five miles southeast of 

 the town of Hay wards, a total length of about twenty-two miles. The 

 lava flow has a maximum residual thickness of about 400 to 600 feet, 

 but generally it is much thinner. It presents a rather distinct physio- 

 graphic feature on the front of the hills due to the steep slopes of its 

 erosional forms. At a number of localities the streams have eroded 

 through the entire thickness of the rhyolite and are now cutting into 

 the older rocks below. Indeed, it seems quite evident that the dis- 

 continuous areas of rhyolite are remnants of a former continuous lava 

 flow that was much thicker and wider. 



NATURE OF SURFACE UPON WHICH LAVA FLOWED 



In some places the rhyolite rests upon the Franciscan, in others 

 upon isolated patches of Knoxville which were not entirely removed 

 from the Franciscan by erosion at the time of its extrusion, so that the 

 lava probably occupied the valley into which it flowed. It was hoped 

 that considerable quantities of stream gravel would be found below 



o Op. ext., p. 20. 



