112 
THE GEAND CANON DISTEICT. 
prevent access of water to the cavities it floats upon water like a cork.* In 
this condition the lava is extremely susceptible to weathering. It quickly 
turns gray or rusty, and dissolves into soil. But in this lava field the most 
delicate pumice is still intensely black, and only here and there may be 
found specimens which begin to show the gray. (Plate XXI.) 
Any attempt, however, to fix the age of these lavas must prove quite 
fruitless. All that we can say is that it is very recent, even when time is 
judged by the historic or human standard. It cannot be many centuries 
old, and it may be more recent than the Spanish conquest. But there are 
reasons why lavas should here preserve for centuries the freshness which is 
lost in other countries in as many decades. The climate is arid, and there 
can be no question that the chemical action of the meteoric agents upon the 
lavas pi’oceeds here with extreme slowness. 
Between the oldest of the middle-aged eruptions and this modern out- 
pour we find lavas of all intermediate ages. No long period within those 
limits appears to have passed without witnessing the activity of some one 
or more of the many cones now standing. 
THE HUEEIOANE FAULT. 
We turn now to the great displacement which forms the western 
boundary of the Uinkaret. Of all the great dislocations of the western 
mountain region there is surely none more wonderful or more interesting. 
Its full extent is not yet known, but the greater part of it has been well 
ctudied. We do not know, as yet, where the southern end of it is located, 
or in what manner it runs out, whether by merging into the great displace- 
ment of the Aubrey Cliff's, or by gradually vanishing in the sierra country 
of Arizona. We know, however, that it appears in the Carboniferous plat- 
form south of the Colorado, and that it extends 30 or 40 miles in that direc- 
tion without undergoing any great modification of the features it presents 
where it crosses the river. This portion of it, however, has not been ex- 
* It may be remarked here that this basaltic pumice differs from ordinary rhyolitic pumice, the 
latter usually having very elongated or tubular vesicles, while the vesicles are seldom drawn out in the 
Uinkaret basalt, but are nearly round or polyhedral. 
