The High Altitude Conoplain -Ogilvie. 33 
a minimum, hence there are no causes to produce the soften- 
ing prominent in an eastern landscape. And when water 
is present at all, it comes in sufficient bulk to produce a tor- 
rent of large volume and high velocity. Such a torrent 
anywhere would cut steep-sided canyons, provided only 
that the rock cut into is sufficiently hard to stand in cliffs. 
In many cases joints are present which cause the rock to 
break off in blocks leaving cliff faces. So in the erosion 
cycle of our laccolith, the plain will not only be scarred at 
all stages, but it will at all stages be cut by steep-sided 
canyons. 
Another interference with the ideal cycle is the wind. 
No one who has seen the whirlwinds moving over the 
deserts of Sonora or Chihuahua can feel any doubt as to 
the great possibilities of wind as an erosive agent. The 
general effect of wind upon a region such as the one under 
consideration would be the removal of fine material, thereby 
lowering the plains, the scarring of the hard rock by 
mechanical abrasion, and the drying of the soil. 
Such may be considered the normal factors of erosion, 
but the cycle in nature is usually interrupted, or has ab- 
normal conditions at the start. Among the interruptions 
may be mentioned vulcanism, the presence of some large 
river flowing to the sea, and climatic changes. For if some 
river is near enough to be reached by the streams, the lac- 
colith at once becomes a part of the drainage basin of that 
river and its cycle is limited by the level of the river, which 
in turn is limited by sea level. Variations in humidity 
would change the position of the point of disappearance, 
and damper epochs would produce rejuvenation. 
The actual laccolith is rarely ideal, but usually consists 
of several different intrusions, not necessarily circular in 
outline, into strata not originally horizontal, the whole more 
or less disturbed by faulting. The Ortiz mountains are 
abnormal in all these respects. It is no part of the present 
paper to describe them, but only to point out the generali- 
ties of this process as exemplified in them. 
If it is possible for a plain to be cut at high altitude 
in the case of a small and isolated laccolith, the question 
at once arises as to whether some similar process may not 
