72 Scientific Proceedings, Royal Dublin Society. 
estuary of the Fuerte river before volcanic disturbances diverted 
its course. The high-tide outline of the top of the bay has the 
appearance of a delta with numerous channels, though there is 
now no fresh water there; and the bay, which is very beautiful | 
and surrounded by steep volcanic hills, made a fitting exit for a 
river having an inland course of great interest and variety. The 
existing mouth ends ignobly in mud-flats infested by alligators ; 
for the river has not had time to make anything better, but it will 
probably improve it in the course of a few thousand years. It 
would be interesting, and probably easy, to trace the old bed of 
the Fuerte, and the work might result in the discovery of more 
placers to reward the pioneer. 
There are indications that the confluence of the Urique and 
Batopilas rivers, with those from Chinipas and Septentrion, was 
formerly a long way from the present fork at La Junta; and that 
the wild gorge from Realito to La Junta was cut by an overflow 
of the volcanic dam at a spot different from the old exit of the 
Urique river. Upon this supposition, the placers near Baca were 
produced by the Chinipas river only. 
The cause of the small volcanic groups outside the edge of the 
table-land appears to lie in the existence of a great shearing stress 
acting on the Harth’s crust at this place. The mesa sloping down 
from 9000 to 6000 feet, and then plunging abruptly to river- 
channels 1000 feet above the sea, causes a sudden change of load, 
due to the weight of 5000 feet thickness of rock; and, as erosion 
changes the position of maximum stress, weak places are disclosed, 
through which any hot fluid matter below is squeezed out, while 
the plateau settles down. This action may have been repeated 
several times while the plateau was being cut back, thus accounting 
for the voleanic hills at Topolobampo, and generally along the 
coast. 
In the clear waters of Topolobampo Bay, pelicans fish day and 
night unceasingly, turtles bask, and porpoises race by at railway 
speed, punctuating the silence of night with their rhythmical 
snorts, while hundreds of thousands of sea-birds feed on the sandy 
beaches of San Ignacio and nest on islands in the bay, watched 
over by the paternal care of the Mexican Government. 
If we depart by sea the last view of the coast isa weird outline 
of fantastic shapes, distorted by a veil of mirage. 
