540 ABSENCE OF TREES. Book III. 
those places where none are now growing. There exists, 
however, positive proof to refute such an opinion. In 
the farthest settlements of the prairies on the Missouri I 
have seen planted Robinise grow as vigorously as in any 
spot in the United States. No trees grew formerly in the 
place on the Texan coast where the town of Galveston 
now stands, and yet at the present time there is no scarcity 
of them in the streets and the gardens of the town (mostly 
Ailanthus), all exhibiting a healthy and vigorous growth. 
The elevation above the sea has nothing to do with the cause 
of this phenomenon as considered under a general point of 
view, since the treeless regions extend from the sea-coast 
on the one side, across the highest summits of the table-lands 
of the central countries down to the sea-coast on the other 
side. The few exceptions from this general character of the 
vegetation are seen at the bottom of valleys, cut into the 
plateau, and upon the highest summit of the mountains 
rising above them. From this fact the conclusion might 
be drawn, that deficiency of moisture is one of the many 
causes that produce the phenomenon, although it cannot be 
the chief cause, for the prairies of lower Texas are as 
little deficient in moisture as the Californian coast. With- 
out entering further into this subject, I wish merely to 
offer here an opinion, suggested by my personal observation, 
that the main cause of the deficiency of trees must be 
looked for in the geological history of these regions, viz., 
in the nature of those circumstances which have clothed 
these regions with any vegetation, and of those which later 
on befel them. I have no doubt that in the course of 
time a great part of the present treeless regions of the con- 
tinent will be clothed with trees. 
Vallecito is a small green oasis, surrounded on all sides 
by barren mountains. The vegetation of the valleys con- 
