Drift Mounds near Olympia^ Wash. — Rogers. 395 
These are doubtless old bottoms of Puget sound, made dry by eleva- 
tion. They are covered with drift soil. These grassy prairies are cov- 
ered as thickly as possible with mounds, about three to four feet high, 
and thirty to forty feet in diameter. There are probably millions of them. 
The general appearance is that of almost perfect regularity of size and 
shape. The soil of the mounds is rather fine drift, with pebbles not 
larger than a pigeon's egg. The intervals between the mounds are 
strewed with larger pebbles. The mounds are occupied by ferns, the 
intervals only by grass. These treeless spaces are called 'mound prai- 
ries' ...... .Erosion removes the finer top soil, leaving it, however, In 
spots. The process once commenced, weeds, shrubs, and ferns take pos- 
session of these spots as the better soil, or sometimes as the drier soil, 
and hold them, and by their roots retard the erosion there. In some 
cases a departing vegetation— a vegetation gradually destroyed by an 
increasing dryness of climate — is an important condition. 
Professor Le Conte argues that the agency of erosion must ac- 
count for these mounds. He starts with the idea that these tracts 
"are doubtless old bottoms of Puget sound, made drj' by eleva- 
tion. " Did subsequent erosion take place? If so, it would ap- 
pear that the newly elevated plain was comparatively smooth, a 
surface stratum of the same material as the present tumuli being 
quite evenly distributed over the entire area. Then "erosion" be- 
gan, etc., as above quoted. At this juncture, I pause to point out 
what seems a grave error in at least one of Prof. Le Conte's affir- 
mations. It would seem that he could not have personall}' exam- 
ined these tumuli, or at least with care. He says: "The soil of 
the mounds is a rather fine drift, with pebbles not larger than a 
pigeon's egg. The intervals between the mounds are strewed with 
larger pebbles." Instead, the tumuli contain as large pebbles and 
cobblestones as those strtiwed between them. They range from 
the size of a pigeon's egg to three, five, or more inches in diameter. 
I think it is quite true, however, that the soil at and near the top 
of the mounds ma}^ be slightly finer, also that the pebbles may 
average a little smaller than those below. In fact one receives 
the impression that there is a gradation, not altogether uniform 
however, from finer material above to coarser below, throughout 
the mass. It is quite safe to affirm that more than half the bulk 
of these mounds is composed of coarse gravel enclosing large peb- 
bles and cobbles, the major part of which are quite the same as 
between the mounds. Hence it is reasonable to suppose that if 
this area was elevated as a smooth plain, its surface was material 
like that of the mounds, evenly distributed. If then the finer mat- 
