MAP OF CLEAE LAKE. 235 



the Knoxville grou[). They occupy the greater portion of the surface and 

 are in great part highl}- plicated and metamorphosed or silicified. On this 

 foundation rest Chico-Ti'jon beds, PHocene strata, and volcanic rocks, with 

 the last of which are associated the quicksilver deposits. 



The distribution of the rocks is shown on the reconnaissance map 

 (Atlas Sheet III). The topography of this map was not prepared for the 

 Survey, the necessity for the examination of so large an area not having 

 been apparent until the detailed examination of the Sulphur Bank had 

 begun. It was compiled by Mr. C. F. Hoffmann from the published work 

 of the former State geological survey, from pints in the surveyor general's 

 office, private snrvevs by Mr. R. K. Nichols, of Lower Lake, the detailed map 

 of Sulphur Bank prepared for this volume, private notes of the compiler, 

 and a little supplementar}^ work by Mr. J. D. Hoffmann. While it makes 

 no pretension to the detailed accuracy of the special maps prepared by the 

 geographical division of the Survey, it represents the country fairly well, 

 perhaps as accurately as the present needs of this section of the State de- 

 mand. The geology is represented npon it as minutely as the character of 

 the map will permit, preference of course being given to the indications of 

 the topography rather than to the bearings of known jtoints wherever tliere 

 was a slight discrepancy. The data are on record for still more accurate 

 plotting, should the preparation of a detailed map ever be undertaken. 



The Knoxville series.— No fossils of the Knoxvillc group were discovered in 

 the immediate vicinity of Clear Lake, bnt the series Avas followed almost 

 without a break to the Manzanita mine on Sulphur Creek, Colusa County, 

 where AiiccUa coiiceutrira. and RhynclioneUa Whifi/rjjl are abundant. Tlie 

 lithological character of the altered rock of Clear Lake, its .structural pecu- 

 liarities, and its relation to the ChicoTtjon series, when compared with 

 those of fossiliferous occurrences elsewhere, all indicate beyond any rea- 

 sonable doubt that it belongs to the Knoxville group. 



The Knoxville beds of tlie Clear Lake region have undergone very 

 violent disturljances. A great deal of time and pains were spent in a S3's- 

 tematic investigation of the dips of these rocks in the area shown on the 

 map of Sulphur Baidc for the pnrpose of constructing sections, but this was 

 found whollv impossible. The entire mass has been shattered, and the 



