266 The Naturalist in Nicaragua 
raise the water for the supply of the town. At the bottom 
are seen great cliffs of massive trachyte (No. 1 in section). 
Above this is an ash bed, then a bed of breccia containing 
fragments of trachyte, then another bed of cinders, which 
looks like a rough sandstone, but is pisolitic, and contains 
pebbles of the size of a bean. This bed is surmounted by one 
that possesses great interest (No. 5 in section). It is com- 
posed of fine tufa, in which is imbedded a great number of 
large angular fragments of trachyte, some of which are more 
than three feet in diameter. It is the last bed but one, the 
surface being composed of lightly coherent strata of tufaceous 
ash, worn into an undulating surface by the action of the 
elements. 
I believe there is but one explanation possible of the origin 
of these strata, namely, that the great bed of trachyte at the 
base is an ancient lava bed; that this, perhaps long after 
it was consolidated, was covered by beds of ashes and scorie@ 
thrown out by a not far distant volcano, and that at last a 
great convulsion broke through the trachyte bed and hurled 
the fragments over the country along with dense volumes 
of dust and ashes. The angular blocks of trachyte imbedded 
in the stratum No. 5 in section are exactly the same in com- 
position as the great bed below, and in them I think we see 
the fragments of the rocks that once filled the perpendicular- 
sided hollow now occupied by the lake. Looking at the vast 
force required to hollow out the basin of the lake, by blasting 
out the whole contents into the air—distributing them over 
the country so that they have not been piled up in a volcanic 
cone round the vent, but lie in comparatively level beds—I 
cannot expect that this explanation will be readily received, 
nor should I myself have advanced it if I could in any other 
way account for the phenomena. Still, within historical 
times, there have been volcanic outbursts, not of such magni- 
tude, certainly, as was required to excavate the basin of the 
lake of Masaya, but still of sufficient extent to show that such 
an origin is not beyond the limits of possibility. 
Thus, in the same line of volcanic energy, not far from the 
boundary line of the States of Nicaragua and San Salvador, 
there was an eruption of the volcano of Cosaguina, on the 
20th of January 1835, when dense volumes of dust and ashes, 
and fragments of rocks, were hurled up in the air and 
