48 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
depart very far from the fine-grained, even, and thin-bedded deposits 
that are appealed to in the Green river formation as convincing evidence 
of lacustrine origin. In the Bridger area, there are abundant alterna- 
tions of shales and sandstones; in the Vermilion creek area, of sand- 
stones and conglomerates. Many of the beds are visibly lenticular and 
local, implying rapid variations in the conditions of origin, horizontally 
as well as vertically. But there is little new in all this, so far as facts 
of structure are concerned. Many details of structure are accurately 
stated in the reports of the Fortieth Parallel Survey. For the Bridger 
formation, we read : along the line of the railroad the lower members of 
the formation are seen “consisting of thin-bedded drab and greenish 
sandstones and clays” (ii. 245). North of Echo canyon, the Tertiaries 
consist “ chiefly of red sandstones containing some fine shale and clay 
beds, and limited sheets of conglomerate” (ii. 331). It is chiefly to the 
interpretation rather than to the record of the facts that renewed 
analytical discussion should be directed. 
It was perhaps natural at the time that the early western surveys 
were undertaken to class all fresh-water basin deposits as lacustrine ; 
but the thirty years that have elapsed since then have as naturally 
introduced new interpretations. The unanalyzed, undetailed theory of 
wholesale deposition in a series of great lakes now seems to be entirely 
untenable for many of the fresh-water Tertiary formations of the Rocky 
Mountain region. The fine calcareous shales of the Green river forma- 
tion strongly suggest deposition in standing water; but the associated 
cross-bedded sandstones imply an activity of movement in the deposi- 
ting agent such as would characterize.a stream rather than a lake. The 
heavy, coarse, and variable conglomerates of the Vermilion creek forma- 
tion suggest deposition by running water; there seems to be no local 
indication of lacustrine conditions. The real need here is a resurvey of 
these formations, in which the facts shall be interpreted in view of all 
possible conditions of deposition. 
