The High Altitude Conoplain— Ogilvie. 31 
and lessened velocity would work together to produce depo- 
sition at a point near the edge of the disturbed area. 
If the rainfall were equably distributed the point of dis- 
appearance of streams would gradually move nearer the 
mountains as more material accumulated. The theoretical 
end of the cycle would come when the laccolith became so 
far reduced that it could no longer catch the moisture, and 
wind alone would carve its surface. This old age laccolith 
would in a general way resemble the mature island ; it would 
have slight elevation, be carved by radial valleys, and 
would be surrounded by a cut plain sloping gently away on 
all sides, this in turn being surrounded by a built plain. 
The whole would be closely analogous to the sea level forms 
of peneplain grading seaward into stratified deposits. 
But in the region under consideration this ideal cycle 
probably never took place, since it would normally be inter- 
fered with by the two factors, unequal annual distribution 
of rainfall and wind. 
The effect of the unequal distribution of rainfall is an- 
alogous to that of an oscillating coast. Given a coast that 
is alternately rising and sinking, no peneplain will be pro- 
duced. If an approximation towards it develops, a slight 
uplift will rejuvenate the streams causing them to incise 
steep-sided channels ; a slight sinking will drown the 
streams and fill their channels with deposits. 
Similar processes are normally going on in the degrada- 
tion of a laccolith. The burning heat of summer pushes 
the point of disappearance nearer the mountains, and even 
most of the springs go dry. At some uncertain period in 
the fall or winter, rains come and then torrents rush down 
rapidly cutting through the previously formed alluvial de- 
posits, redepositing them farther out on the plain. These 
mountain torrents often change their courses entirely from 
one season to the next, the course depending upon the more 
or less fortuitous arrangement of the surrounding alluvial 
material. Therefore the surrounding conoplain is deeply 
scarred with arroyos and there are more arroyos than are 
ever full at any one time. Hence in no stage of the actual 
erosion cycle is the conoplain absolutely flat. In all stages 
it will be cut by gullies, but surface inequalities will be 
largely obliterated by filling with alluvial deposits. 
