The High Altitude Conoplain—Ogilvie. 29 
such types as Pinusponderosa (var. scapulorum), Quercus 
undulata, and various shrubs of the oak and holly families. 
Associated with these are the cactus-like types, Opuntia r 
Cereus and Yucca. The vegetation is thus, for an arid re- 
gion, a considerable one. The surrounding plains present 
a marked contract; Opuntia, Cereus and Yucca, together 
with Artemisia (sage brush) form the prevailing types. 
The largest trees there are pinon and a small cedar. 
The result of this difference in climate is that the moun- 
tain springs give rise to streams which disappear entirely 
a short distance from their source. Within the Ortiz area 
there is no permanent stream which finds its way to the 
sea. For the greater part of the year the arroyos are en- 
tirely dry, and many of the springs dry up also. But when 
rain comes, it comes in quantities, and a few days of storm 
will start raging torrents. 
The 'details of the idea of the growth of river valleys 
and of cycles of erosion have been developed in regions of 
moderate climate and of equably distributed rainfall. It is 
evident that in such a region as the Ortiz the normal erosion 
cycle will be markedly different. Leaving aside for the 
moment the larger question of the origin of the great 
plateaus, and also the special case of the Ortiz mountains, 
let us consider the theoretical erosion history of an ideal 
laccolith. 
If we imagine a symmetrical laccolith of homogeneous 
rock, to have arched up the strata of previously horizontal 
rocks, the initial stage of erosion may be compared to that 
of Prof. Salisbury's homogeneous, symmetrical island. But 
with this difference that in the case of the island the limit 
in down-cutting is a result of checked velocity and is at sea 
level ; in the case of the laccolith the limit is formed by the 
point at which the streams disappear, and may be at any 
altitude. 
The transporting power of a stream depends upon 
volume and velocity. An increase in volume increases the 
transporting power by more than a simple ratio ; velocity 
depends upon volume and declivity ; hence an increase in- 
volume indirectly adds to the transporting power by in- 
creasing the velocity. And an increase in declivity aids the 
