TLIOCENE DEPOSITS. 239 



of it is included in the map has not been ascertained. These beds consist, 

 first, of conglomerates, carrying pebbles of metamorphlc rock identical 

 with tliat which underlies them, and of pyroxene-andesite which cannot be 

 discriminated from that of Chalk Mountain; secondly, of s uid beds; and, 

 thirdly, of arg-illnceous and calcareous deposits. For the most part the strata 

 are but little compacted and may be reduced to powder in the hand ; but there 

 are frecjuenth' nodular masses which are consolidated to firm rock. Some of 

 the blulfs of conglomerate — for example, those in Grizzly Canon ^are stud- 

 ded witli such nodules, distributed somewhat uniformly over the surface. 

 Elsewhere single strata of sand or clay are petrified, and occasionally, as 

 on Perkins's Creek, considerable areas of sandstone fully solidified are met 

 with. The impression conveyed by the prevalent distribution of the more 

 extended and irregular, hardened masses is that they represent the local 

 action of cold, calcareous or siliceous waters upon the surrounding rock, an 

 action which, if sufficiently prolonged, would result in the complete pet- 

 rifaction of the whole system of beds. A similar effect of mineral springs 

 on recent deposits ma3^be seen at sj3veral points in the district, particularly 

 along Sweet Hollow Creek. The isolated nodules cannot be produced in 

 this way and, like those in the Chico of New Idria, they are probably due to 

 the decomposition of oiganic matter, as explained in Chapter III. 



The Cache Lake beds have been subjected to comparatively little dis- 

 turbance. They are tilted at angles varying from 10° to about 4(P, but tlie 

 inclination seldom changes rapidly, and there is very rai-ely anything which 

 can be regarded as contortion. Within the area of the map, too, no faulting 

 was traced, thougTi more or, less important disturbances of this nature 

 occur near Chalk Mountain and oiTTtlS north fork of Cache Creek, east of 

 the map limit. The thickness indicated by measuring the strata perpen- 

 dicularly to the planes of stratification is very great — some thousands of 

 feet. I confess myself unable either to comprehend this or to ignore its 

 significance. There is certainly no confusion between these beds and 

 others of marine origin, since fresh-water shells were found in them at 

 widely separated horizons; but the accumulation of several thousand feet 

 of sediment in any lake except one of vast dimensions seems an impossi- 

 bility. A careful search was made for faults without finding any. The 



