NON-COXFOUMITY BELOW THE CHICO. 189 



ever, no exposures sliowing both series together coukl be found from wliieh 

 thoroughly satisftictory inferences could be drawn as to the relations of the 

 underlying and overlying rocks. 1 therefore resorted to a study of the 

 exposures of each separately, for which the region offers unusual facilities. 

 It was found possible to follow single strata of the Chico uninterruptedly 

 for the greater part of a mile, and, by the aid of lithological peculiarities, 

 combined with topographical indications and the strikes observed at the 

 exposures, to recover the croppings with substantial certainty after passing 

 -intervals covered with detritus. The contact with the metamorphic rocks 

 was also laid down and numerous dips were observed in the metamorphic 

 area. In order to eliminate the disturbing influence of the irregularities of 

 the topography, the croppings of each of these continuous strata and the 

 contact of the metamorphic were reduced to their intersections with normal 

 planes cutting the surfaces respectively at the mean elevation of their ex- 

 posures. The results showed that adjoining Chico strata are parallel and 

 thrown into extremely gentle undulations, while the metamorphic area is 

 merely a shattered mass The contact has approximately the same general 

 direction as the Chico beds, but does not entirely coincide with their strike. 

 It is a rouo-h line, but not rougher than one which would represent the ver- 

 tical section of an ordinary sea-bottom near tlie coast. The dip of the Chico 

 strata decreases as the distance from the contact increases. 



Either this structure represents a nonconformity or else the metamor- 

 phism and accompanying disturbances occurred after the deposition of the 

 Chico beds, but ended sharply at a certain line. It might at first sight seem 

 impossible that an area several miles in width should be crumpled and broken 

 quite as thoroughly as a representative area of the Archaean along the east- 

 ern coast, the rocks being also for the most part converted into serpentine 

 and chert, and that, nevertheless, both mechanical and chemical action 

 should cease abruptly at a given hue. Yet instances, of which the above 

 might pass for a description, actually occur and are perhaps more frequent 

 in the Coast Ranges than elsewhere. All geologists who have visited this 

 region are aware of the very irregular distribution of the metam.irphic 

 areas, and it has already been pointed out that the metamorphic rocks pass 

 over into unaltered or very slightly altered Knoxville beds suddenly, though 



