274 The American Geologist. November, ioos 
that the terra •'structural valley" or "structural trough" was 
used in reference to the large features of the topography as 
distinguished from simply valleys of erosion cut mainly by 
stream action. The very essential feature of a bolson is 
that the plain is bordered by mountain forms or plateau 
escarpments. The mountains may be of the fold or fault 
type, but rising as they do above the general level' of the 
plateau upon which they stand the intervening area might 
properly be considered a structural valley. In some cases 
this intervening area may be so protected from erosion by 
the distribution of the mountain uplifts that it will be pre- 
served and will present the structural characters of the 
original plateau plains. Again it may be in the line of the 
great longitudinal drainage lines of the plateau and be large- 
ly removed by subsequent erosion. As far as the later 
formation of a bolson is concerned, it would appear that 
either type might properly be called a structural valley, and 
with the later deposition of the detritus forming the bolson 
plain, there would be a striking difference in the thickness 
of the deposits, those in the eroded structural valley being 
much thicker. 
It would appear also that in the formation of such a 
structural valley by the enclosure of a portion of a plateau 
by the elevation of bordering mountains., where the valley 
is not subjected to subsequent erosion and the remaining 
valley floor is fairly horizontal, there would be produced a 
topographic form which would resemble very closely the 
bolson but which would be as essentially different from it 
as an aggradation plain is different from a peneplain. 
These two forms having similar superficial characters may 
be easily mistaken, the one for the other, for with the eleva- 
tion of the bordering mountains enclosing a structural valley 
free from erosion, there will most certainly develop around 
the margin of the remnant of the enclosed plateau plain, 
talus hills and fan cones and frontal wash aprons which will 
rapidly work out over the floor of the valley and eventually 
convert it into a bolson plain while the superficial charac- 
teristics may remain almost the same during the entire pro- 
cess. 
It is apparent that when the filling of the valley takes 
