AGE OF THE METAMORPniCS. 183 



that as a rule the hills of metamorphic rock are synolinaV f^'id consequently 

 they must have undergone great erosion. The elevations of later age do 

 not exhibit this peculiarity. 



Rocks of the metamorphic series often pass over into unaltered beds in 

 the Coast Ranges under such circumstances as to leave no doubt that they 

 are of the same age ; but unfortunately the unchanged strata seldom con- 

 tain determinable fossils and onl}^ a small number of occurrences is known 

 in which the age can be satisfactorily established by direct evidence. In 

 addition to these cases, however, there is a considerable amount of tolera- 

 bly satisfactory indirect evidence available, when all the circumstances are 

 taken into consideration. The neighborhood of Knoxville affords an ex- 

 cellent opportunity for the study of the metamorphic rocks. The section 

 across the north fork of Davis Creek, a little north of the Reed mine, a short 

 distance from Knoxville, shows that the ravine occupies an eroded anticli- 

 nal, of which the western portion is highly metamorphic, while the eastern 

 consists in part of highly fossiliferous strata containing Aucella of two 

 varieties, with other moUuscan remains characteristic of the horizon which 

 in this memoir is called the Knoxville series. The geological map of the 

 district shows that the strike of the unaltered strata throughout is tolerably 

 constant, but that areas of metamorphic and unaltered rocks, the latter 

 nearly all containing a few fossils, are interspersed iu the most irregiilar 

 mannei'. \Yliile the passage from metamorphosed to fresh rock is usually 

 rather sudden, there are also clear cases of transition. The whole structure 

 and the stratigraphical relations are such as to preclude everj^ hypothesis 

 except one, viz, that the metamorphic rock is an alteration product of the 

 same beds which contain Aucella and the accompanying fossils. 



Close to the Manzanita gold and quicksilver mine on Sulphur Creek, 

 in Colusa County, the metamorphic rocks contain impressions of Aucella 

 Pioch'd, and close by are beds of limestone full of Rhynchonella Whitneyi? 

 The metamorphic rocks of this region are serpentinized and silicified, and 



'Also ob'served by Professor Whitney : Tbe Auriferous Gravels, Mem. Mus. Comp. Zool. Harvard 

 Coll., vol. 6, No. 1, 1880, cbap. 1. 



'Tbese specimens were determiuej by direct comparisoa with spacimens iu tbe collection of tbe 

 State survey. Tbe figure given iu Geol. Survey California, PaliEontology, vol. 2, PI. XXXIV, is incorrect 

 in important particulars. 



