GREAT EASTERN DISTRICT. 363 



Consequently their age is a matter of inference. The facts bearing on this 

 question are as follows: Along the coast of Sonoma County, to the south 

 of Ft. Ross and a little more than six miles from the Great Eastern, I found 

 a series of unaltered sandstone beds lying unconformably upon the meta- 

 morpliic series. The overlying strata are fossiliferous at some points and 

 have been named the Wallala beds. Dr. White has determined the fossils 

 which they contain as ]\Iiddle Cretaceous. The underlying metamorphic 

 series is older, and there can be little doubt that it is at least as old as the 

 Knoxville series. It may possibly be older, but all the characteristics of 

 the rock are -absolutely identical with those of the material at numerous 

 points from Colusa County to San Luis Obispo, in which AuccUa has been 

 found. There is, furthermore, nothing in this part of the country suggest- 

 ing the presence of strata earlier than the Knoxville series. So far as there 

 is any evidence as to the age of the rocks at the Great Eastern, therefore, 

 they are to be regarded as Neocomian. 



Quicksilver rock.— Ill tliis district thcro are nun>erous occurrences of opal- 

 ized rocks. Of these many are small, seemingly isolated patches. In two 

 cases this material forms defined ledges, standing up from the surface on 

 account of the resistance which it offers to decomposition and erosion. 

 These ledges strike nearly east and west magnetic, which seems to be the 

 prevalent strike of the strata also. In one of these are the deposits of the 

 two mines. At the surface the metalliferous ledge is nearly vertical, but 

 at lower levels it dips to the north. It lies between a hanging wall of sand- 

 stone and a foot-wall of serpentine. 



Ore deposits. — It wlU be remembered that at the Great Western also a 

 layer of opalized rock lies between serpentine and sandstone. At the Great 

 Eastern, however, the ore is inclosed in the dark, opaline mass, instead of 

 being adjacent to it. The ore body was continuous from the surface to the 

 lowest workings, a vertical distance of 4.50 feet. The ore does not form a 

 nearly vein-like sheet in the ledge, but an irregular pipe, the axis of which 

 is inclined to the horizon at an angle of about 50°. So far as it has been 

 developed it is entirely embedded in the opalized rocks and does not touch 

 either the sandstone or serpentine. The ore does not appear to have been 

 deposited simultaneously with the amorphous silica, but in openings in tlie 



