324 PAST CLIMATES AND CLIMAXES. 



show, too, that this quantity of dust yearly during a period of 100,000 years 

 would produce a layer over the earth only about a half a millimeter, or one- 

 fyftieth of an inch, thick, and therefore one could hardly expect to find any 

 marked accumulation of it, even if it had filled the atmosphere for much longer 

 periods. (32) 



"It has been shown in the above, Among- many other things, that volcanic 

 dust ia the high atmosphere decreases the intensity of solar radiation in the 

 lower atmosphere and therefore the average temperature of the earth, substan- 

 tially as theory indicates a priori that it should; and this effect has been clearly 

 traced back to 1750, or to the time of the earliest reliable records. Hence it is 

 safe to say that such a relation between volcanic dust in the upper atmosphere 

 and average temperatures of the lower atmosphere alwa,ys has obtained, and 

 therefore that volcanic dust must have been a factor, possibly a very important 

 one, in the production of many, perhaps all, past climatic changes, and that 

 through it, at least in part, the world is yet to know many another climatic 

 change in an irregular but well-nigh endless series — ^usually sHght though 

 always important, but occasionally it may be, as in the past, both profoimd 

 and disastrous." (34) 



Humphreys's table (plate 57) of the relation of pyrheliometric values and mean 

 temperature departures to sun-spot nmnbers and violent volcanic eruptions 

 is convincing evidence of the striking effect of the latter. It also shows clearly 

 the cumulative effect of the coincidence of sun-spot maxima and volcanic 

 dust, as in 1767^1770, 1785-1788, 1816, 1837, etc. Thus there would seem 

 little doubt that the continuance of such conditions for many years wo\ild 

 suffice to bring on an ice age. But the only evidence of such continuance in 

 geological times would have to be sought in the coincidence, or immediate 

 sequence, of cold or cooled climates with periods of great eruptive activity. 

 While some evidence of this nature occurs in the cooling foimd in the Meso- 

 phytic era and at the beginning of the Cenophytic, the present state of knowl- 

 edge warrants the conclusion of Schuchert (1914 : 287, 258) that — 



"We may therefore conclude that volcanic dust in the isothermal region of 

 the earth does not appear to be a primary factor in bringing on glacial climates. 

 On the other hand, it can not be denied that such periodically formed blankets 

 against the sun's radiation may have assisted in coohng the climates during 

 some of the periods when the continents were highly emergent." 



Huntington (1914 : 258; 1914^: 483, 544) agrees with Schuchert, and assumes 

 that — 



"The volcanic hypothesis is of importance at particular times. So far as 

 the past 3,000 years are concerned, however, there seems to be no good reason 

 for assuming that its importance has been any greater than during the last 

 30 years. The recorded volcanic eruptions show no apparent relation to the 

 climatic changes indicated in the California curve. If there had been volcanic 

 eruptions sufficient to cause the pronounced pulsation which figure 14 shows to 

 have occurred, between 1300 and 1500 a. d., it seems scarcely credible that 

 they should have attracted little attention. We can not assert this positivdy, 

 however, for certain parts of the world where volcanoes are now important 

 were not then known, and their history is not recorded even by tradition. Our 

 chief reason for beheving that the volcanic hypothesis is of only minor impor- 

 tance is that this appears to be its position to-day." 



