MESOZOIC OF SOUTHWESTERN OREGON 551 



THE SHASTA (LOWER CRETACEOUS) SEA 



It has been shown that the Myrtle group terminates to the north 

 (in the Myrtle Creek area) against a Franciscan territory, with a 

 conglomerate over a thousand feet thick at its base. This con- 

 glomerate contains abundant Franciscan detritus. Some of the 

 chert may possibly be derived from pre-Franciscan terranes, but this 

 remains to be proved. Almost nothing that must be referred to the 

 pre-Franciscan was found. No pebble of any of the older schists 

 was observed, and only a very rare one of granite. As this heavy 

 conglomerate can be traced for over twenty miles on the northern 

 limit of the Myrtle, and is thinner on the south flank of the syncline, 

 it may be inferred that the shore-line of the early Knoxville sea was 

 not far distant, with land, made up of Franciscan surface exposures to 

 the north. 



The occurrence of Horsetown overlapping the Knoxville and 

 resting unconformably, often with basal conglomerate, directly on 

 the pre-Knoxville terranes, indicates a subsidence and transgression 

 of the sea during the Shasta period. This general relation has been 

 pointed out by Diller, who has shown that in the northern California 

 field, "everywhere beyond the limit of the Knoxville beds, the Horse- 

 town beds rest with a marked unconformity, directly on the meta- 

 morphics." 1 He has even been to able show, by the aid of fossils, 

 that the upper part of the Horsetown overlaps the lower. 



As the Shasta in the Oregon region here discussed is limited by 

 the Klamath Mountains on the south, against whose older terranes 

 it rests with basal conglomerates, we may conclude that the Knox- 

 ville sea entered this region as an arm elongated in an east-west 

 direction, and that with the passing of time the sea transgressed 

 farther and farther, the arm becoming broader, especially toward the 

 south and southeast, until it probably, as also believed by Diller, 

 united with the waters encroaching from south across northern 

 California around the other side of the Klamath Mountains. 



The present investigation has, in particular, made it possible to 

 trace more closely the nature and boundaries of the Myrtle arm of 

 the Shasta sea, and to infer the nature of the rocks which formed the 

 coast line. 



1 Diller and Stanton, Bulletin of the Geological Society 0} America, Vol. IV (1893), 

 p. 214. 



