ELEVATION AND DEPRESSION OF EARTH'S CRUST. 133 



of the earth, the effect of the moon on the meridian would be to assist, 

 and on the horizon to repress, any force whatsoever tending to break 

 up the crust of the earth and to produce an earthquake. 



Relation of Earthquake-Occurrence to Seasons and Atmospheric 

 Conditions. — By extensive comparison of earthquake-occurrence with 

 the seasons, it has been shown that they are a trifle more frequent in 

 winter than in summer. Constructing a curve representing the annual 

 variation of earthquake-intensity, this curve rises to its maximum in 

 January, and sinks to its minimum in July. But the difference is 

 small. 



Prof. Knott has recently suggested what seems a possible expla- 

 nation of this, at least for Japan, where this relation is quite marked. 

 During winter, there is high barometer — i. e., great atmospheric pressure 

 over the whole of Northern Asia (Siberia), and low barometer over 

 equatorial Pacific. In addition to this, the heavy winter snow-fall 

 greatly increases the pressure over Siberia. In summer, the condition 

 of things is reversed — the barometer is low and the snow is removed 

 over Siberia, and the barometer is high over mid- Pacific. This 

 change of excess of pressure from a large land-area to a large ocean- 

 area back and forth, must tend to fracture the earth-crust, or to pro- 

 duce readjustment of previous fractures, along their dividing line, i. e., 

 along the margin of the sea-basin. This is known to be the place of 

 origin of most of the Japanese earthquakes. This would take place 

 mainly in winter, if the tendency of the earth-forces producing earth- 

 quakes were to produce readjustment by subsidence of land or elevation 

 of sea-bottom. 



There is an almost universal popular belief in earthquake-regions 

 that the occurrence is preceded by a still, oppressive state of the air. 

 Although no scientific investigations have confirmed this impression, 

 yet it seems quite possible and even probable that diminished atmos- 

 pheric pressure, indicated by a low state of the barometer, may act as 

 a determining cause of earthquake- occurrence, precisely as the position 

 of the moon on the meridian. In both cases, however, we must regard 

 these not as true causes of earthquakes, but only as causes determining 

 the moment of occurrence. 



Section 4. — Gradual Elevation and Depression of the 

 Earth's Crust. 



Of all the effects of. igneous agencies these are by far the most im- 

 portant. Although not violent and destructive like volcanoes and 

 earthquakes, although indeed so little conspicuous as to be generally 

 unobservable except to the eye of science, yet acting not paroxysmally 

 but constantly, not in isolated spots but over wide areas and affecting 

 whole continents, their final result in modifying the crust of the earth 



