VOLCANIC ASH IN IOWA 
51 
boulders containing numerous small logs of charcoal^ which evi- 
dently were once drift-wood. 
The occurrence of the ash-bed imediately beneath the Wiscon- 
sin till-sheet and directly upon the Yarmouth-Sangamon-Peorian 
loess, which was wind-formed, fixes the time of deposition as also 
interglacial — probably Peorian. This brings the date of deposi- 
tion very close to us, geologically speaking. 
In casting about for the location of the nearest possible source 
of such volcanic material we naturally look first of all to the 
Southwest which would be the direction of the prevailing winds. 
The nearest volcanic eruption of the violent kind that we know of 
is Mt. Capulin, in northeastern New Mexico. This is an ash cone 
whose magnitude far surpasses that of the famed Vesuvius. It 
is 3000 feet high and its crater is half a mile across. But Capulin 
is very recent. Its last eruption perhaps scarcely antedates the 
landing of Columbus on the western continent. There are other 
volcanoes in the Capulin field which might have served, for volcan- 
ic outburst there was continuous throughout Quaternary and part 
of Tertiary time. Spanish Peaks, in southeastern Colorado, seem 
too early in their last eruptions. Another possibility is the San 
Francisco group of volcanoes in northeastern Arizona, where 
more than 400 vents of all sizes and many dates occupy a circum- 
scribed area. Volcanoes of the Pacific coast are all almost out of 
question. 
Although this appears to be the first announcement of an oc- 
currence of volcanic ash in Iowa it is perhaps not nearly so unu- 
sual a phenomenon as might be inferred. Now that the exact 
stratigraphic horizon is determined numerous other deposits may 
be expected to be speedily found. Because of the fact that the 
ash beds of the kind under consideration are eolian deposits their 
positions are unaffected by the ordinary means of deposition. 
Then, too, the determination of the exact age enables certain sim- 
ilar beds in Nebraska, as noted by Todd, ^ to be placed and corre- 
lated. With the western extension of these correlations we may 
confidentially expect at no distant day the tracing of the deposits 
to the exact vent of eruption. 
The volcanic ash as seen under the microscope (figure 2) ap- 
pears as sharp, angular fragments of clear, amorphous glass, whol- 
ly without trace of crystal structure. The dust is quite character- 
istic of the pumaceous glasses found in ash cones commonly built 
around volcanic vents. Associated with the glass-dust in the same 
1 Science, Vol. VII, p. 373, 1886. 
