EARTHQUAKES AND VOLCANOES. 
403 
rupted only on the southeast by the narrow gorge 
through which the waters of the lake are discharged, 
it receives no important affluents from the surface; and 
as its emissary is of much greater volume at all seasons 
than these insignificant brooks, it is probably fed by sub¬ 
terranean springs, — indeed one of these, near the south 
shore, enters with such force as to cause a ripple on the 
surface of the lake. Soundings indicate a cup-like bottom 
with an extreme depth of less than seven hundred feet 
(209.26 metres). The level of the lake has often changed, 
and in 1880 the surface-level fell more than thirty-four 
feet, leaving exposed stumps of trees encrusted with cal¬ 
careous deposits. It was before the last eruption well 
stocked with fish of the varieties called by the people 
who lived near by mojavra , burrito (both species of the 
