2To. l.] HOLMES ON FOSSIL FORESTS OF YELLOWSTONE PARK. 131 
As to the character of the seas or lakes in which the Yolcanie Tertiary- 
beds were laid down, it is clear that their waters were fresh, hut 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 Yolcanie 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 full 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 Y olcanic Tertiary rocks, however, the upward movement 
had ceased. 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 
Canon, 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 wliilu 
