Cranclall — Santa Clara Valley Region in California. 49 



The type section of Horsetown, near the place of that name, 

 is considered by Mr. Diller to represent only the upper part of 

 the horizon.* 



Dr. L. F. Ward has lately identified plant remains from 

 Horsetown, which he has pronounced Lower Cretaceous in age.f 



It is evident that the Horsetown is an independent horizon. 



Distribution of the Horsetown Horizon. — The Horsetown 

 has long- been known in northern California, but has not been 

 recognized in central or southern California. A few species 

 representative of this horizon which have been found in the 

 vicinity of Mt. Diablo, have been referred to the Chico. X Dr. 

 Becker has expressed the opinion that there was an unconformity 

 between Chico and Knoxville at Mt. Diablo, § and Dr. Fair- 

 banks I has described these formations as unconformable in 

 southern California. 



Mr. Turner has found a conformable series of 10,000 feet 

 of Cretaceous at Mt. Diablo with Aucella Piochi at the bottom 

 and lower Chico forms near the top.^f There are five thousand 

 feet of unfossiliferous beds in between these two places, which 

 Mr. Stanton thinks must represent Upper Knoxville and the 

 Horsetown. Sedimentation was slower at Mt. Diablo than 

 in northern California, or else Mt. Diablo was out of water 

 during the Horsetown period.** 



All of these statements have tended to the general ojrinion 

 that the Horsetown was absent in the Mt. Diablo region and 

 that Knoxville and Chico are unconformable. 



From the Santa Clara Valley region there are forms which 

 show the probable presence of Horsetown at certain localities. 



The fauna, given in the table, shows the possible presence 

 of Horsetown at Pleasanton, Arroyo del Valle, Haywards, 

 Berkeley, Benicia and Mt. Diablo. Further proof of the pres- 

 ence of this horizon at the last-named place is, that there are 

 five thousand feet of beds between lower Knoxville and 

 lower Chico, and field work has shown the whole Cretaceous 

 series to be conformable here, as is true at Arroyo del Valle 

 and Pleasanton. The thickness of Chico Turner gives as six 



*Geol. Cal. and Oregon, Bull. Geol. Soc. Am., iv, 213. 



f Mesozoie Flora of the U. S. Monog. xlviii, U. S. Geol. Sur., Pt. I, p. 227. 



% T. VV. Stanton, Fauna of the Knoxville Beds, Bull. 133, U. S. Geol. Sur- 

 vey, p. 21. 



§ Stratigraphy of Cal., Bull. 19, U. S. G. S., p. 16. 



|| H. W. Fairbanks, Pre-Cretaceous Age of Metamorphic Rocks of the Coast 

 Eanges. Am. Geol., ix, 165; also Stratigraphy of Calif. Coast Eanges, Jour. 

 Geol., iii, 426. 



IT Geol. of Mt. Diablo, Bull. Geol. Soc. Am., ii, 401. 



**T. W. Stanton, Fauna of the Knoxville Beds, Bull. No. 133, IT. S. G. S., 

 p. 21. 



Am. Jour. Sci.— Fourth Series, Vol. XXIV, No. 139. — July, 1907. 

 4 



