190 QUICKSILVEU DEPOSITS OF THE PACIFIC SLOPE. 



under circumstances wliicli preclude tlie supposition that the adjoining ureas 

 represent different formations. There are, liowever, significant differences 

 between these occurrences and the conditions at New Idria. The limits of 

 nietamorphism in areas consisting of Knoxville beds, however sharp they 

 may be, are exceedingly irregular, the outline being substantially inde- 

 pendent of stratification, cutting strata more often than following them and 

 presenting all sorts of convolutions; there are almost invariably also out- 

 lying areas of metamorphic rock and included masses of unaltered rock; 

 furthermore, at least here and there, distinct transitions occur between un- 

 altered and metamorphic rock. At New Idria, on the other hand, a section 

 of the contact normal to the surface extends over at least several miles (as 

 far as it was followed) in a tolerably persistent general direction. There 

 are no outlying patches of metamorphic rocks; the included masses of 

 comparatively unaltered rock seem wholly different from the Chico strata 

 above them, and, though there is a considerable alteration of a portion of 

 the overlying mass, this alteration is not of the same character as the mag- 

 nesian and siliceous metamorphism of the underlying rock; nor could I 

 find anv distinct case of transition. Finally, as has already been men- 

 tioned, in the Chico conglomerates a part of the pebbles entirely resemble 

 the silicified or jasperj' portions of the present metamorphic area, while a 

 few are both macroscopically and microscopically indistingui.shable from 

 the serpentinized rocks of Knoxville age 



Some further evidence of the relation of the two series was found a 

 few miles to the southeast of New Idria, where a branch of Cantua Creek 

 cuts through a portion of the range. Here the heavy-bedded, tawny Chico 

 sandstones, lying at an angle of about 30°, cap the hills wliicli are inter- 

 sected by the brook, while in the bed of the stream the thin-bedded, met- 

 amorphic strata stand vertically. No actual contact, however, could be 

 found, the interval being covered with detritus. 



Tliere seems no reasonable explanation of the structure at and neai 

 New Idria, except on the theory of a non-conformity. Though the evi- 

 dence may seem less satisfactory than that which would be presented by an 

 ideal exposure, it is derived from the correlation of the structural evidence 

 aloii"' the contact for nnles, and in this respect is superior to any but that 



