CLIMATES OF GEOLOGIC TIME—SCHUCHERT. 289 
colors, and these are the deposits of the times when the oceans have 
most widely transgressed the lands, and therefore the times of greater 
humidity. The maximum of continental extension falls in with 
red deposits and more or less arid climates. (See curve for aridity 
in fig. 4, p. 305.) 
VOLCANIC DUST AS A CLIMATIC FACTOR. 
Two interesting papers on the subject of volcanic dust as a 
climatic factor have recently appeared. These articles, which are 
by W. J. Humphreys,! should be read by every student of paleo- 
meteorology. The following are the conclusions reached: 
[Volcanic dust in the upper atmosphere has been one of] several contributing causes 
of climatic change, * * * a cause that during historic times has often been fit- 
fully operative, and concerning which we have much definite information. * * * 
At an elevation that in middle latitudes averages about 11 kilometers the tempera- 
ture of the atmosphere becomes substantially constant, or, in general, ceases appre- 
ciably to decrease with increase of elevation, this is, therefore, the upper limit of 
distinct vertical convection and of cloud formation. Hence, while volcanic or other 
dust in the lower or cloud region of the atmosphere is quickly washed out by snow 
or rain, that which by any process happens to get into the upper or isothermal region 
must continue to drift there until gravity can bring it down to the level of passing 
storms. In other words, while the lower atmosphere is quickly cleared of any given 
supply of dust, the isothermal region retains such dust as it may have for a time that 
depends upon the size and density of the individual dust particles themselves, or 
upon the rate of fall. * * * Volcanic dust once in the upper atmosphere must 
remain in it for many months and be drifted out, from whatever origin, into a thin 
veil covering perhaps the entire earth. * * * A veil of volcanic dust must pro- 
duce an inverse greenhouse effect, and if long continued, should perceptibly lower 
our average temperature. Let us see then what observational evidence we have on 
the effect of volcanic dust on insolation intensity and average temperatures. 
. Pyrheliometric records [show] that there was a marked decrease in the insolation 
intensity from the latter part of 1883 (the year this kind of observation was begun) to 
and including 1886, from 1888 to 1892, and during 1903. There has also been a similar 
decrease since about the middle of 1912. Now all these decreases of insolation inten- 
sity, amounting at times to 20 per cent of the average intensity, followed violent vol- 
canic eruptions that filled the isothermal region with a great quantity of dust. * * * 
It appears quite certain that volcanic dust can lower the average temperature of 
the earth by an amount that depends upon the quantity and duration of the dust, 
and that it repeatedly has lowered it certainly from 1° F. to 2° F. for periods of from 
a few months to fully three years. Hence it certainly has been a factor, in determin- 
ing our past climates, and presumably may often be a factor in the production of our 
future climates. Nor does it require any great volume of dust to produce a marked 
effect. Thus it can be shown bya simple calculation that less than the one-thousandth 
part of a cubic mile of rock spread uniformly through the upper atmosphere as vol- 
canic dust would everywhere decrease the average intensity of insolation received 
at the surface of the earth by at least 20 per cent and therefore would, presumably, if 
long continued, decrease our average temperatures by several degrees. * * * This 
1A summary paper appeared first, entitled ‘‘ Volcanic dust as a factor in the production of climatic 
changes,” Jour. Washington Acad. Sci., 3, 1913: 365-371. The complete article is “ Volcanic dust and 
other factors in the production of climatic changes, and their possible relation to ice ages,” Bull. Mt. 
Weather Obseryv., Washington, 6, Pt. I, 1913, 1-34. 
73176°—sm 191419 
