UNCONFORMITY. 295 



containing no fossil from the New Idria Cliico beds, which seem to prove tliat 

 the induration is due to the decomposition of an organic nucleus. 



When the rock is of nearly uniform texture spherical concretions often 

 extend over several lamina-, the regularity of which is undisturbed by the 

 induration of the nodule. In other cases the concretions are flattened, and 

 this form seems to be related to the unequal composition of adjoining beds. 

 'Fhe shales of this formation are remarkable for nothing but their rarity. 

 Conglomerates are still less frequently found, but such a bed is exposed on 

 the road from the furnaces to the upper mine at a distance of a few hundred 

 feet from the contact with the metamorphic mass. 



Non-conformity.— This is au occuiTeHce of mucli importance, for the pebbles 

 which it contains are metamorphic and are composed of siliceous and ser- 

 pentinoid rocks. Such rocks must therefore have been exposed to erosion 

 at the time when the lower portion of the Chico was deposited, and probably 

 at no great distance from the position now occupied by the conglomerate. 

 There is no I'eason to suppose that the metamorphic rocks were other than 

 those now exposed, and the metamorphism with the attendant upheaval must 

 consequently have taken place prior to the deposition of the Chico, or, in 

 other words, there nuist be a lack of conformity between the two. 



Before the pebbles of this conglomerate had been studied under the 

 microscope I had made out the existence of the non-conformity on structural 

 grounds, as was described in Chapter V. It is unnecessary to repeat that 

 argument at length here. 



The Chico beds constitute a conformable series, inclined at an angle 

 which is high close to the contact witli the metamorphic rocks and dimin- 

 ishes as one proceeds northward. A result of tlie steepness of the contact is 

 the absence of any single exposure by which a non-conformity can be 

 proved beyond a doubt ; but a study of the relations along the whole line 

 leads at least as satisfactorily to the conclusion that there is a want of con- 

 formity. Over nearly the entire area mapped the Chico strata strike along 

 gently undulating lines whose minimum radius of curvature is usually about 

 one mile, and this regularity is persistent up to the contact. Only at the 

 western edge of the map is there considerable disturbance among beds of 

 the Chico series. 



