No. 1.] HOLMES ON FOSSIL FORESTS OF YELLOWSTONE PARK. 131 



As to the character of the seas or lakes in which the Yolcanic Tertiary- 

 beds were laid down, it is clear that their waters were fresh, hnt as to 

 their extent or distribution little is known. The formations cover or 

 have covered an area of not less than 10,000 square miles, but they lie 

 at a much greater elevation above the sea than the formations of syn* 

 chronous lakes of neighboring provinces, and, so far as is known, have 

 no actual connection with them. 



It has been suggested by some one that these coarse volcanic strata 

 may have been formed in very restricted bodies of water held high 

 amongst the mountain ranges ; but lakes cannot exist without barriers, 

 and as has already been shown, the conglomerates, although naturally 

 disintegrating more rapidly than any of the older rocks, now form the 

 summits of many of the highest peaks that face the eastern plains, and 

 the basins in which they were formed must have had free communication 

 with the lowlands to the west, from the beginning to the end of the 

 period. 



For my present purpose it is sufficient to know that the bodies of 

 water of this period were of sufficient extent not to be greatly affected 

 in level by the filling-in of volcanic products or by the oscillations of 

 the district under discussion, since we can have no correct measure of 

 those oscillations of the surface which define the thickness and decide 

 the character of strata without the barometer-like records of a sea-level. 



The change of level produced by the great oscillation that preceded 

 the Volcanic Tertiary period, and brought the lofty ranges of this region 

 into existence, cannot fall far short of 20,000 feet. In order to reach these 

 figures, we have but to add to the fidl thickness of the palaeozoic and 

 mesozoic strata the present elevation of the granitic ranges above the 

 lowest observed stratum of the Tertiary rocks. At the beginning of the 

 deposition of the Yolcanic Tertiary rocks, however, the upward movement 

 had teased. The land had undergone enormous erosion, and subsidence 

 had commenced. The great ranges that had lifted their crests to such 

 lofty heights were again sinking beneath the sea. This subsidence did 

 not cease until all, or nearly all, of the mountain peaks were submerged. 

 It is in the strata deposited during this great subsidence that we must 

 look for evidences, of conditions and events that made the entombment 

 and preservation of a vertical mile of forests possible. 



The Yellowstone Yalley, from the head of East Fork to the Lower 

 Caiion, is carved out of strata which were formed along the west and 

 south bases of the main eastern range of mountains. In many places 

 the river has penetrated the full thickness of Tertiary strata, and has 

 cut down into bodies of metamorphic rocks that at the beginning of the 

 age were promontories or islands. It is plain, therefore, that those 

 parts of the tree-bearing strata examined, were deposited along a shore- 

 line, or, at least, near the borders of the Tertiary lake. Over large dis- 

 tricts there must have been, during the period of general subsidence, a 

 frequent alternation of land and sea. Land would have to exist while 



