118 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
With reference to the large boulders, whose transportation is ascribed 
by Johnson to river ice or uprooted trees, the observations of Stone go 
to show that such agents are not required to account for the presence 
of the boulders. He cites examples of cloudbursts in Colorado where 
the streams produced were successively dammed by hail, then broke 
through the obstructions with rapid current and transported slabs of 
sandstone four feet square and two feet thick. One such stream 
200 to 300 feet wide and 20 feet deep (in the deepest place) issued 
from the narrow valley of Templeton's Gap near Colorado Springs. 
It became wider on the plain but was swift enough to carry the above 
mentioned boulders one-third of a mile. Previous to the flood the 
plain at this point was composed of sand loosely grassed over. The 
boulders were dropped upon the sand plain which was but little eroded 
by the swift currents; then, as the flood slackened, sand was deposited 
upon and around the boulders to the depth of from one to three feet. 
Numerous similar observations in Colorado show boulders of consider- 
able size surrounded by fine sand and gravel or even embedded in clay. 
It thus appears that swift currents can flow over a stratum of fine sedi- 
ments having an even or level surface without eroding much. This 
is largely due to the fact that the lower part of the water is nearly 
stopped by friction. The stream cannot get at the sediment while 
it remains coherent. Stone considers it certain that large stones and 
even boulders may be deposited by running water in the midst of sedi- 
ments as fine as sand, oreven in clay. The requisite is a rapid cur- 
rent moving over aneven surface and acting for a rather short time. 
(Stone, p. 17-18). 
An interesting case of deep fluviatile deposits is that of the Great 
Valley of California described by Ransome. Borings in the San 
Joaquin Valley at Stockton have penetrated 2,000 and 3,000 feet 
respectively without reaching the bottom of the unconsolidated 
fluviatile sediments. The latter consist of fine sands and clays with 
occasional beds of coarser sand and gravel (Ransome, p. 381) but 
there appears to be no definite gradation in size of materials vertically. 
Delta Deposits. Where rivers carry sediment into lakes or into 
those parts of the ocean where the action of currents, waves, and tides 
is insufficient to sweep away the material delivered by the streams, 
deltas are formed. An extended account of their surface and struc- 
tural features, geographical distribution, classification, and origin, 
together with the effects of elevation and subsidence of coast lines is 
given by Credner. More recently Gilbert has described their general 
characteristics in connection with other features displayed by the 
