Vol. 6] Merriam: Virgin Valley and Thousand Creek. 



225 



correlation purposes. In no other region, and at no other geo- 

 logic horizon, do we know a series of igneous outflows exceeding 

 in magnitude and in areal extent the Miocene lavas in and con- 

 tiguous to the Columbia River area. For purposes of reference 

 in correlation this series would seem to furnish a most important 

 datum plane wherever it can be traced, or wherever recognized 

 by any petrographic peculiarities. 



Following the deposition of the early Miocene lava flows, 

 conditions favorable for the accumulation of sediment obtained 

 in many areas over the lava-covered regions, and extensive de- 

 posits were formed, of which presumably a large part have since 

 disappeared through erosion; but patches and even extensive 

 areas have remained in many places. During this period the 

 rich and varied mammalian fauna would presumably distribute 

 itself with unusual uniformity over the wide stretch of territory 

 which had been occupied in the period immediately preceding 

 by the great lava flows. It is to be presumed that the large lava 

 areas, covering more than 250,000 square miles, would permit 

 a particularly wide distribution of certain forms at that time. 

 Though the lava beds appear to have been subjected to disturb- 

 ance in some regions, the amount of movement was probably not 

 sufficient to raise barriers which would offer important obstacles 

 to the distribution of most mammalian forms. Judging by what 

 we know of the mammalian faunas referred to the Middle 

 Miocene of the West-American province, there was actually a 

 notable uniformity in the life over this region during this epoch. 



The distribution of the Miocene flows which seem to be 

 related to the great sheets poured out in the Columbia River 

 region has not been determined with exactness. Nevertheless 

 one seems to be justified in certain suggestions as to the probable 

 extension of this field to the north and south of the Columbia. 



To the north of the Columbia, the lavas seem to be traced with 

 certainty in eastern Washington, and upon them is found a sedi- 

 mentary series known as the Ellensburg formation, which re- 

 sembles in its general character the Mascall of Oregon. Such 

 fossil remains as have been reported from these beds, particu- 

 larly the plants, correspond to those of the Mascall. 



To the south of the Columbia, the lava fields extend around 



