1917] Nomland : The Etchegoin Pliocene of Middle California 201 



decided change of conditions taken place between the time of depo- 

 sition of the two groups of deposits. That the Santa Margarita 

 corresponds to a part of the San Pablo farther north more closely 

 than to the Etchegoin becomes evident from a comparison of the 

 faunal lists in the present paper. 



In the region south and west of Coalinga the Etchegoin overlies a 

 formation decidedly different lithologically from the coarse sand- 

 stone of the Santa Margarita north of that town. This formation 

 consists predominantly of a compact, resistant shale which has been 

 referred questionably to the Santa Margarita. In many localities, 

 with excellent exposures along Waltham Creek and Kreyenhagen 

 Hills, this shale appears to grade into the overlying sandstone. As 

 a consequence the contact line has been placed only approximately. 

 This is shown by the work of Pack and English, 18 who state: "No 

 evidence of unconformity between the diatomaceous shale of the Santa 

 Margarita ( ?) formation and the sandy beds of the upper Miocene 

 was found, the line of contact being drawn at the horizon where the 

 shale becomes sandy rather than clayey and weathers yellowish brown 

 rather than purplish." The line of demarcation is placed somewhat 

 lower by Arnold and Anderson, 19 according to whom: "There is a 

 non-fossiliferous transition zone of alternating sandstone and shale 

 beds having a thickness of several hundred feet, which may belong 

 to either formation. This has been tentatively included with the 

 Jacalitos in mapping, the beds to which the name Santa Margarita 

 formation is applied being restricted for the present to the underlying 

 shale." 



In the process of investigation of the Etchegoin an unconformity 

 has been found that may mark the contact between that group and 

 the Santa Margarita formation in the area west and southwest of 

 Coalinga. This unconformity was first discovered at about the middle 

 part of the southern boundary of sec. 7, T. 22 S, R. 15 E, M. D. B. & 

 M., at an elevation of about 2100 feet. Prom this point it was traced 

 in a west-northwest direction to a point at which it crosses the western 

 boundary of the same section a little north of the middle of the south- 

 west quarter, at an elevation of about 1600 feet. The contact con- 

 tinues in the same general direction until it reaches the eastern bank 

 of Jasper Creek. No attempt was made to follow the unconformity 

 to the southeast beyond the point of discovery. The exposures of the 



"XL S. Geol. Surv. Bull. 581D, p. 132, 1914. 

 i»U. S. Geol. Surv. Bull. 398, p. 92, 1910. 



