66 Scientific Proceedings, Royal Dublin Society. 
cliffs from 40 to 70 feet high. Against the sky is the vast wall of 
the distant mesa, indented with square notches, like the machico- 
lated parapet of a castle, by the straight-sided ravines. 
The bed of the ancient lake is 1000 feet above sea-level 
at its lowest point, and consists of white and bright green. 
sandstones and conglomerates in thin but continuous layers. 
During the dry season the river bed is a convenient road, though 
it necessitates frequent fording, as the stream meanders from side 
to side. The sandstones were examined for fossils, but none 
were found. After nine miles of gently-sloping, walled-in course, 
a place is reached where the lake beds have been cleared away 
nearly down to the present river level: probably by wanderings 
in the course of the current: and on this plain is the small town 
of T'ubares. 
The ancient lake and its fluctuations of level must have been 
caused by volcanic eruptions in the previous river valley damming 
the channel up with lava and ash; and when largest, the lake 
covered 100 square miles, to a maximum depth of at least 1000 
feet ; the thickness of the lacustrine strata below the present river 
level could not be ascertained without boring. 
Should it ever be desired to regulate the flow of the Fuerte 
river, either for water-power or for extended irrigation in the low 
country, it could be done efficiently at a moderate cost, by con- 
structing a dam at the gorge of Realito, where there is a narrow 
passage, solid rock for foundation, and a long gentle slope for the 
reservoir. 
Above Tubares, low cliffs close in again on the river ; here they 
are of a compact white stone, in appearance like limestone, but 
probably derived from the waste of the acid lavas ; then the valley 
opens again as the junction with the Urique river is reached, the 
left branch being the product of the main stream from Hl Zapori 
and the Batopilas river. 
Long ages elapsed between the eruptions that built up the 
mesa, and those which formed the Realito group of foot-hills; for 
during that interval, the Urique and other rivers eroded valleys 
more than 5000 feet deep, and extended the littoral plain at the 
expense of the plateau. After the new volcanoes had covered the 
plain, the rivers deposited the lacustrine beds of Tubares, and cut 
a deep channel through the erupted material. 
