620 
he regards as fluviatile. And on my return 
home from a sight of the Plains this sum- 
mer, the receipt of Monograph XXVII., 
U.S. G.S., on the Geology of the Denver 
Basin, by Emmons, Cross and Hldridge, 
suggests a further extension of the discus- 
sion. Itis particularly with regard to the 
Arapahoe and Denver formations, in Colo- 
rado, that the conditions of origin seem 
open to another interpretation than that 
given by these authors ; for it is noticeable 
that a lacustrine origin seems to have been 
almost taken for granted, and that a fluvia- 
tile origin is not discussed. 
The Arapahoe formation is thus intro- 
duced: ‘‘ After an erosion of the Laramie 
beds * * *, a considerable fresh-water lake 
was formed and sedimentation again set in. 
What the exact area of this lake was it is 
not possible now to determine; * * ** whether 
the lake was continuous along the mountain 
front or there were several small isolated 
basins it is as yet impossible to deter- 
mine. * * * In it were deposited more 
than 600 to 800 feet of sediments. * * * 
Of these sediments the lower 50 to 200 
feet were conglomerates, the upper 400 
to 600 feet arenaceous clays. Vertebrate 
remains are found in both the conglomer- 
ates and the clays, more abundant and bet- 
ter preserved, however, in the latter.” (P. 
31, 32.) 
The interval between the Arapahoe and 
Denver formations is thus described: ‘“ Be- 
tween the deposition of the Arapahoe and 
Denver beds a considerable time-interval 
occurred, during which, as the record of the 
rocks shows, the Arapahoe lake was drained 
and the sediments deposited in its bottom 
were considerably eroded. The movement 
which caused the drainage of the lake was, 
as far as present indications go, rather local 
in its effects, and produced no important 
deformation of the beds already deposited. 
* -k * This movement was succeeded, after 
a considerable lapse of time, by a depression 
SCIENCE. 
[N..S. Vou. VI. No. 147. 
sufficient to allow of the formation of a 
second lake in the Denver basin. * * * The 
nature of the depression which produced 
such lakes without admitting marine waters 
to any extent within the area affected is not 
readily conceivable, yet its effects are shown 
to have been widespread by the considerable 
thicknesses of fresh-water beds consisting 
largely of eruptive débris.” (P. 32.) 
The following description is given of the 
Denver deposits : ‘‘The beds deposited in 
the Denver Lake reached a thickness of 
over 1,400 feet along the flanks of the 
mountains, but were probably somewhat 
thinner toward the middle of the basin. 
* * - That the Denver beds were depos- 
ited in shallow waters is shown by the fre- 
quent cross bedding observable both in 
sandstone and conglomerate, and by the 
plant remains and standing tree stumps 
that abound at certain horizons. * ** The 
Archean material contains large boulders, 
and the sand grains are angular.” (P. 
33.) 
The vertebrate paleontology of the Den- 
ver basin is treated by Professor Marsh in 
the later pages of the monograph. Nearly 
all of the typical vertebrate fossils of the 
Denver region here discussed ‘‘ were essen- 
tially land animals, but not a few of them, 
especially of the Reptilia, lived near the 
water and there met their fate. The preser- 
vation of their remains was probably, with- 
out exception, due to their entombment be- 
neath the waters of the great fresh-water 
lakes which existed in this region during 
Mesozoic and Cenozoic time.”’ (P. 525.) 
The plants discussed by Knowlton are land 
plants, not lacustrine. The table of inver- 
tebrate fossils (p. 78,79) gives none to the 
Arapahoe beds, and only six to the Denver 
beds, all of which imply ‘ fresh water, ’ and 
most of which suggest, according to my 
colleague, Dr. R. T. Jackson, a fluviatile 
rather than a lacustrine origin. 
Now waiving for the time all reference to 
