28 



KNOWLEDGE & SCIENTIFIC NEWS. 



[February, 1907. 



shell, being insufficiently supported, caved in liere 

 and there, permitting the great fissure eruptions which 

 produced those great tracts which we call the lunar seas. 

 These extensive outilows ol la\a dissolved the original 



Fi^. 3,~Bullialdus. 



solid shell whenever they came into contact, obliterating 

 its features. The same obliteration of smaller craters (on 

 a much reduced scale) can be perceived in Hawaii. In 

 all probability the same thing, Professor Pickering sug- 



¥t' 



\y^- 



gests, tODk place on our own Earth on a larger scale 

 instead of on a small one. The inner core of the Earth 

 shrank still more from the cooling and hardening outer 

 crust ; the resulting cataclysms were still greater and 

 more destructive; and through great fissures there were 

 enormous outllows of lava and molten rock, now recog- 

 nised as "archaic rocks," which conipleteiy dissolved and 

 destroyed the gigantic blister craters which once studded 

 our globe. 



The volcanoes of Southern l£urope, which are those 

 most completely studied, have little in common with those 

 of the Moon. In the case of the volcano of Vesuvius, for 

 example, a high truncated cone has been built up by 

 mild eruptions of steam and cinders, sometimes alternat- 

 ing with lava. At long intervals violent explosions 

 occur, which sometimes blow away a large portion of 

 the summit. Such an explosion occurred when Pompeii 

 was overwhelmed, and it was repeated on a minor scale 

 last year. The most violent explosion of the kind of 

 which we have any record was that which occurred not 

 at \'esuvius but at Krakatoain 1883. Nothing whatever 

 of that kind is perceptible among the discoverable craters 

 of the Moon. In volcanoes of the engulfment type, as 

 opposed to the explosive type, comparatively little steam 

 is evolved ; often there is no exterior cone, and the craters 

 enlarge quietly by the cracking off, and falling in of their 

 walls. This species of crater is to be found in Hawaii, 

 though the Hawaiian structures are on a comparatively 

 small scale, the largest of them being one-hundredth the 

 diameter of a lunar crater. 



On the great seas or vuiyia of the Moon, secondary 

 engulfment craters were formed ; and of these Pessel, 

 which is about twelve miles in diameter, is a large and 

 well-known example. There are no great craters of that 

 size on the Earth to compare with it ; all the Earth's 

 largest craters being of the explosive type.''' At 

 Hawaii, with the exception of the three great craters of 

 Haleakala, Mokuaweoweo, and Kilauea, few of the crater 

 pits exceed half a-mile in diameter ; but it is possible to 

 compare these with those of the Moon, in spite of the 



discrepancy 

 of size. On 

 the Earth at 

 present, the 

 cooling pro- 

 cessas wit- 

 nessed at 

 Hawaii al- 

 ways inter- 

 venes before 

 great size 

 IS attained. 

 Formerly, 

 the lava was 

 hotter when 

 it issued 

 from the in- 

 terior; more- 

 over, the 





Fij;. 4. — Kauhaku, Molokai. 



* The three 

 greatest cra- 

 ters on the 

 liarth, about 

 fifteen miles 

 in diameter, 

 occur in Kam- 

 ch a tka, in 

 Japan, and in 

 t lie I'h i lip- 

 pines. All are 

 Lf ihe explo- 

 sive type. 



