August 28, 1908] 



SCIENCE 



281 



and -will be equivalent to a condition of uplift 

 throughout the mass, fissures will tend to 

 widen, and gases in magmatic material will 

 expand. The extreme of this effect will, there- 

 fore, tend to favor the emission of gases from 

 volcanic magma, to determine, the explosive 

 crises of an eruptive period and to bring about 

 the culmination of great eruptions. 



The minimum luni-solar influence will be 

 produced when the sun and moon are in 

 quadrature with the earth, with zero declina- 

 tion (equinox) and with the moon in apogee. 

 But it should be noted here — and this is a most 

 important point — that this celestial minimum 

 constitutes a terrestrial maximum. The 

 virtual neutralization of the luni-solar distort- 

 ing power gives full sway to terrestrial 

 re-formation, the results of which, although 

 the converse of those we have just considered, 

 are fully their equal in importance. These 

 periods of maximum terrestrial action will 

 result in a general increase of lateral 

 pressures, tending to the compression of 

 fissures and to the breaking of strata in a 

 condition of stress. The increased pressure 

 on the firepoeket and lava-filled fissures of a 

 volcano will force the magma to a higher 

 level, bringing up fresh lava at a higher 

 temperature and tending to cause lava flows; 

 it will readily be seen how slight a com- 

 pression of a fissure 40 km. in depth would be 

 required to upforee a large amount of lava 

 into the crater of a volcano. Powerful ex- 

 plosive effects may also accompany this phase, 

 being due to the upforcing into the conduit 

 and crater of active, high-temperature lava, 

 and, although of a different character from the 

 paroxysmal gaseous emission of the luni-solar 

 maxima, these explosions may be more effect- 

 ive in rupturing the cone because acting 

 ^against the pressure of a high lava column; 

 they are, therefore, productive of lava flows 

 and often initiate in this way the progressive 

 reactionary process which will lead up to a 

 eatastrophal culmination coinciding with the 

 succeeding luni-solar maximum. As the term 

 luni-solar minimum would be confusing as ap- 

 plied to a condition which produces a positive 

 effect, I prefer to designate this phase by the 



term " terrestrial maximum " as contrasted 

 with " luni-solar maximum." We may, 

 therefore, classify the two orders of maxima 

 with their effects as follows : Luni-solar 

 maxima equals op-position or conjunction plus 

 perigee plus declination, tending to precipitate 

 explosive crises, paroxysmal emission of gases 

 and the culmination of great eruptions; and 

 terrestrial maxima equals quadrature plus 

 apogee minus declination, tending to cause 

 lava flows, rupturing explosive effects and 

 earthquakes. While earthquakes may be 

 caused by either order of maxima, it will in 

 general be found that the great tectonic earth- 

 quakes follow, as we should expect, a ter- 

 restrial maximum, while those of the volcanic 

 or intervolcanic'' type may succeed either order 

 of maxima. Earthquakes lag, as a rule, a day 

 or two behind the culmination of the maxi- 

 mum. As the two orders of maxima are com- 

 plementary, it is obvious that the greatest pos- 

 sible effect will be produced when strong ones 

 occur in proximity, i. e., when a very favor- 

 able luni-solar maximum is followed by a very 

 favorable terrestrial maximum, or vice versa. 

 We may even suppose, given the general 

 ascensional tendency of the volcanic magma, 

 that a sort of pumping action may take place, 

 a terrestrial maximum forcing lava upwards, 

 a luni-solar maximum holding it there by 

 gaseous expansion, a renewal of the terrestrial 

 effect upforcing more lava, etc. It is possible 

 that such an action may play an important 

 role in the formation of new volcanoes, in 

 the production of eccentric eruptions, in the 

 reestablishment of communication between 

 fire-pocket and crater through an obstructed 

 conduit and especially, perhaps, in the erup- 

 tive processes of all trachy-andesitic volcanoes 

 with their viscous, highly silicious magma. 

 The compression and elongation stresses due 

 to these alternating effects may also be a 

 potent source of heat. 



In plotting a curve of the two orders of 

 maxima I have traced these above and below 

 a medial zone representing an interval be- 



*Mercalli thus classifies the Calabrian earth- 

 quakes, believing that these result from the move- 

 ments of deep-seated magma. 



