1916] 



Dickerson: Tejon Eocene of California 



415 



tude, but simply that from what we know of the living species, some or 

 perhaps all of them may have done so. That is, there are none of these 

 genera confined to high elevations. On the other hand there are a number 

 of genera as Alnus, Betula, Populus, Salix, etc., that we would hardly ex- 

 pect to find in great abundance at a very low altitude, but like the last 

 mentioned genera, they are not exclusively confined to high altitudes. They 

 might well have grown on land in close proximity to a body of water 

 connected with the sea, such as the area under discussion appears to have 

 been during their deposition. There are also several genera of conifers, 

 as Pinus, Araucarioxylon, Cupressinoxylon, Pityoxylon, which alone would 

 appear to argue a greater elevation than appears to have prevailed. If 

 these genera were in great abundance, it would still further strengthen 

 that view, but on the contrary they are very rare, the genus Pinus being 

 represented by a single somewhat doubtful example. From their lightness 

 and well-known power of long resisting the processes of decay, they might 

 well have been transported from long distances, for with the exception of 

 the pine-needles there is no evidence to show that any of them grew where 

 they were fossilised. 



From these facts Diller 63 concluded "that during the Miocene 

 the country was a series of plains and peneplains with low mountain 

 ranges ; or in other words, the country was but little above its base 

 level of erosion". 



Lindgren's 64 very careful work on the Tertiary topography of 

 the Sierra Nevada alters this conception considerably and places 

 before us an indisputable body of facts. 



The evidence available shows conclusively that at the time when the 

 oldest gravels, probably of Eocene age, began to accumulate the Sierra 

 Nevada was a mountain range as distinct if not as high, as at present. The 

 rivers headed near the points where the corresponding 7nodern rivers 

 begin now, in a region of lofty peaks and ridges. In geologic literature 

 it has been repeatedly stated that the Tertiary surface of the Sierra Nevada 

 is that of a peneplain, a conclusion absolutely at variance with the opinions 

 of those who have actively studied the range. 



The Tertiary topography of the western slope consisted of four units. 

 Along the valley line extended a number of greenstone ridges attaining 

 elevations of 1,500 to 4,000 feet. They are shown perfectly plainly in 

 Yuba and Butte counties, for instance, by Brown's Valley Ridge and the 

 Oregon Hills, through which the Yuba river of Tertiary time broke through 

 in a deep canyon. In Placer County an area of softer granodiorite reached 

 the valley and in this vicinity — for instance, near Rocklin — the idea of a 

 peneplain is more nearly realized than elsewhere. In Calaveras County, 

 the Tertiary Calaveras River broke through this barrier in a deep valley 

 similar to that of the lower Yuba but much more abrupt; near San An- 

 dreas, for instance, these greenstone ridges rose 2,000 feet above the river 

 in a deep canyon. In Placer County an area of softer granodiorite reached 

 prominently emphasized in the Penon Blanco Ridge and Bullion Mountains. 



as mem, p. 422. 



64 Lindgren, Waldemar, The Tertiary Gravels of the Sierra Nevada of 

 California, Professional Paper 73, U. S. Geol. Surv., p. 37. 



