122 GRAND CANON DISTRICT. 



spread out thinly over extensive surfaces, but as the number of super- 

 posed sheets is considerable in this part of the plateau, the aggregate 

 thickness, though somewhat roughly inferred, must be three or four 

 hundred feet, and occasionally much more. There is not much to add to 

 this description. The lava is apparently all of one kind, but some of 

 it much older than other portions. In truth, it soon becomes apparent 

 that the period of volcanic activity was a long one. A few miles from 

 the point where we attained the summit of the plateau and in a north- 

 west direction from it, we come upon the termination of a lava stream 

 which has the appearance of being extremely recent. It looks as fresh 

 as the emanations from Vesuvius or^Etna which have outflowed within 

 the last fifty years. Its surface is intensely black, and only here and 

 there can we perceive that weathering has even impaired its freshness. 

 Two miles away is seen the cone from which it emanated. The last 

 eruptions from it have almost destroyed it, and melted down the greater 

 part of its mass. 



Skirting the edge of this lava-sheet, we find at the eastern base of 

 Mount Logan a small spring, named the Oak Spring. It is a central 

 point, from which the southern part of the plateau may be visited. 

 There is another very small spring high up on the southwestern side of 

 Mount Trumbull, and its waters have been brought down by a wooden 

 pipe to the plain below, to supply the wants of a saw-mill. A third and 

 much larger spring is found on the western side of the Uiukaret. These 

 are the only available sources of supply, and each may be used as occa- 

 sion requires for the examination of different parts of the plateau. 



It will be necessary here to advert, with the greatest brevity, to the 

 facts which the Uinkaret presents. Its most conspicuous subject is its 

 volcanism. Almost as striking a subject is the great Hurricane fault, 

 which forms the western boundary of the plateau. It also presents 

 many other features of interest, but only the briefest allusion to them 

 can be made here. 



The lavas of the Uinkaret are all basaltic, and are quite typical of 

 their class. They appear to vary but little in their constitution, and, 

 so far as the cursory examination hitherto made indicates, the only differ- 

 ences are such as are incident to varying conditions under which they 

 solidified after eruption, or to subsequent weathering. But it also 

 appears that the period of volcanic activity has covered a considerable 

 duration of geological time. There are old lavas and young lavas ; per- 

 haps we may say there are middle-aged lavas. The older lavas are 

 presented in the largest masses, the largest individual coulees. Another 

 noteworthy feature is that the oldest lavas are now found upon the 

 summits of the loftiest portions of the plateau, while the younger lavas 

 are found chiefly on the lower levels. It is well worth studying to see 

 how this comes about. The facts and explanation are best presented in 

 the fine mass of Mount Trumbull. 

 This mountain is in reality a gigantic butte ; that is to say, a mass of 



