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University of California Publications. [Geology- 



the Blue Mountains and cover the Oligocene John Day forma- 

 tion. In the valley of the John Day River near Dayville the 

 Mascall Beds lie in a trough formed by the Columbia Lava 

 faulted down against the older formations on the northern flank 

 of this portion of the mountain mass. On the summit of the 

 mountains the Columbia Lava appears again dipping gently to 

 the south, where it seems to disappear beneath a formation re- 

 sembling the Mascall. These beds contain a mammalian fauna 

 similar to that in the Mascall on the northern flank of the 

 mountains. 



To the south of the Blue Mountains lies the extensive lava 

 region in which the basaltic flows have been compared by Waring 

 and others to the Columbia Lava, and upon a southern extension 

 of an igneous series comparable to these flows rest the Virgin 

 Valley Beds with a fauna similar to that of the Mascall. 



It does not seem to the writer to be an absolutely safe con- 

 clusion that all of the igneous rocks included in the flows to 

 which reference has been made are necessarily the exact equiva- 

 lent of the main exposures on the Columbia River, or to this 

 series as limited to the basalt flows which lie between the John 

 Day Upper Oligocene and the Mascall Middle Miocene. Other 

 igneous series both earlier and later are known, but there is a 

 reasonable presumption in favor of considering the group of 

 flows to which reference has just been made as belonging to the 

 same general epoch. This epoch on the basis' of correlation by 

 mammalian palaeontology is referable to the Lower Miocene, as 

 the beds immediately below it contain an Upper Oligocene fauna 

 and those immediately above it a Middle Miocene fauna. 



To the south of the Virgin Valley area in Nevada, the 

 broken structure of the Basin region makes difficult the tracing 

 of formations which are not quickly recognized by palaeonto- 

 logic or petrographic species. There are, however, in this region 

 exposures of beds which have superficially the appearance of 

 the Miocene formations farther north, and which contain scat- 

 tering remains of mammalian forms apparently later than early 

 Miocene and older than Pleistocene. Exposures of this nature 

 extend well through the state of Nevada, and may reach into the 

 southern part of California. It is probable that a careful study 



