146 



Prof. J. Prestwich. 



Such are the hydro-geological conditions of a volcanic mountain in 

 a state of rest. The effects when that equilibrium is destroyed will 

 now be discussed. 



§ 5. Condition of the Underground Waters during an Eruption. 



So long as the volcano remains in a state of rest, so long will the 

 hydro- geological conditions described in § 4 continue unchanged. The 

 level, I, of the underground waters may rise ; but, as in the meantime 

 cooling has, by causing solidification, isolated the molten lava, no 

 result is produced except that arising from the trickling of the 

 surface-waters on the yet hot surfaces, giving rise to the small 

 columns of steam common during the periods of rest. After a pro- 

 longed period of repose, even these minor effects cease. It may 

 happen that if the crater is very deep and there has been a' long interval 

 since the last explosion, that the water-level may mount into the 

 crater and give rise to a lake. In most cases, however, such lakes 

 are due to the relatively low position of the crater with reference to a 

 part of the adjacent ground, or to the decomposition of the lava, 

 whereby it has become retentive of the rainfall. Such bodies of 

 water are important in the event of any fresh eruption. 



~Fi&. 4. — Crater Lake dependent on the general water-level (I). 



Crater lakes are not so common or of so large a size in Europe as 

 in Central America. The lake of Atitlas, 1558 metres above the 

 level of the Pacific, is 20 X 15 kilometres large, and of a depth yet un- 

 ascertained. The crater lake of Masaya in Nicaragua is 8 kilometres 

 across, and 150 metres deep in the centre. In 1852 this lake suddenly 

 appeared as though boiling, and a violent explosion shortly followed. 

 The volcano of San Salvador is 2300 metres high, and at the bottom 

 of its crater, which is 700 to 800 metres in diameter, and 400 to 

 500 metres in depth, is a very deep lake. Another large crater lake, 

 12 kilometres west of San Salvador, is level with the ground and 

 200 metres deep.* 



The crater lakes, however, are an exceptional feature. The great 

 body of the rainfall becomes stored within the mountain itself, and 

 makes itself visible only by springs on the lower slopes of the 



* " Voyage Geologique dans la Republique de Guatemala et de Salvador," par 

 MM. Dollfus et de Mont-Serrat, 1868, pp. 103, 106, 318-20, 371-5. 



