164 



SCIENCE. 



[N. S. Vol. XXIV. No. 606. 



sures whielL could become critical and 

 cause flow. 



During the cooling of a globe of heated 

 and partly fused matter, such as the earth 

 and moon are believed to have once been, 

 the early volcanic outbursts on them would 

 be without violence, and it seems likely that 

 the materials ejected would have tempera- 

 tures not greatly above that of the sur- 

 rounding masses. An eruption at that 

 time would be a welling up of great bodies 

 of rather quiescent lava derived at once 

 from the fluid contents below a but slightly 

 cooled surface. As the further cooling 

 made the body more solid and rigid, espe- 

 cially at the outside surface, the mechanical 

 energy spent in causing an eruption would 

 lead to higher temperatures of erupted 

 matter, which would escape with more 

 violence, explosions of vapor or gas would 

 occur, partly due to vapors evolved from 

 the melted masses upon relief of pressure 

 at the vent, and partly the result of contact 

 of the hot lavas with colder surrounding 

 rocks containing gases or water. Ashes 

 and cinders would become a feature of the 

 eruption. At still later stages eruptions 

 would become less frequent and vents less 

 numerous, but the violence of the out- 

 bursts would increase, owing to the very 

 high temperatures reached by the extruded 

 matter during the journey towards ex- 

 pulsion. 



It may seem somewhat paradoxical to 

 hold that as a body, such as is the moon, 

 cooled, the temperature of the ejected vol- 

 canic matter would rise, but in the present 

 view this would be a consequence of the 

 necessary exertion of greater pressures to 

 cause eruption, owing to the enormous 

 amount of mechanical work or energy ex- 

 pended, as the matter would be forced 

 from greater depths. It happens that upon 

 the earth much of the evidence of volcanic 

 action in archaic or early geological times 

 has been obliterated by erosion and sedi- 



mentation, but we have in the moon's sur- 

 face a record which is not so modified or 

 defaced. 



From the whitish streaks surrounding 

 such craters as Tycho Brahe and Coper- 

 nicus, a type of crater not very numerous 

 on the moon, which streaks are most con- 

 spicuous at the time of full moon, it is seen 

 that there have been effects of eruptions of 

 these great craters which extend over hun- 

 dreds of miles from the craters. These 

 whitish streaks are superimposed upon, 

 and therefore later than, many of the other 

 craters and markings over which they 

 spread. These streaks are probably the 

 depositions from condensed vapors which 

 left the craters in almost radial lines. In- 

 asmuch as the moon has very little atmos- 

 phere even at its lower surface and much 

 less at the heights from which these vapors 

 escaped in leaving the craters, the straight 

 line or radial direction taken by them is 

 not surprising; in fact, it is necessary. 

 Vaporous matter escaping into a vacuum 

 moves in precisely that way. Had the 

 earth no atmosphere, the ashes and vapors 

 from a volcanic crater would move away 

 from it in just the same manner, and 

 would be scattered in radial streaks from 

 the notches in the crater walls and to 

 enormous distances. If again the surfa<3e 

 of the moon, as seems likely owing to 

 absence of water, be covered with loose 

 masses, or be broken instead of smooth, or 

 be covered with pieces of rock of varying 

 size, it is easy to understand why these 

 whitish streaks should require for visibility 

 a high angle of solar illumination. The 

 condensed deposits would fall in between 

 the boulders or stones on the surface and 

 at low solar elevations would be in the 

 shade or shadow of the broken pieces on 

 the surface. That the surface of the moon, 

 in large part, may be covered with frag- 

 ments of rock, boulders and stones is prob- 

 able from the great part that volcanic ac- 



