276 GEOLOGY OF THE HIGH PLATEAUS. 



whether they were derived from conglomerates or from rocks in situ farther 

 up the gorge was uncertain. 



A considerable number of basalt fields are found upon the surface 

 of the Awapa. In no instance was any considerable mass of this lava 

 encountered, and wherever found it formed only a rather thin local veneer 

 rarely so much as 100 feet thick, and generally much less. These basaltic 

 sheets have been greatly ravaged by erosion, and their fragments scattered 

 far and wide. No trace of a basaltic cone or monticule was anywhere 

 seen. If any such ever existed it has been totally demolished. I incline 

 to the opinion that none were ever built in those portions of the Awapa 

 which were visited, but rather that the basalt quietly outflowed in the same 

 manner as it did from some of the very recent vents on the Markagunt. 

 From the orifices it seems to have spread out at once in thin, diffuse pools 

 or lakes, where it has slowly weathered away. Wherever it occurs, it is the 

 most recent of the eruptive masses. None of it belongs to so late an epoch 

 as those of the Markdgunt or the southern terraces overlooked by the Pink 

 Clifis. In some localities the sheets of basalt are wasted to mere heaps of 

 disjointed blocks, thickly strewing the platform and partly buried in soil. 

 In others, the continuity of the sheets is tolerably well preserved. It is 

 impossible to fix the age of those eruptions, though I infer that none are as 

 old as Middle Pliocene ; perhaps not so old as the close of that age. They 

 seem to have been erupted after the movements of displacement which 

 blocked out the plateau had well advanced, and these are held to be 

 among the most recent events of Tertiary time. 



I have not attempted to delineate these basalts upon the geological 

 map, being uncertain as to their extent and outlines. 



RABBIT VALLEY. 



The slopes of the Awapa all converge towards a central depression 

 called Rabbit Valley. The trachytic beds descending towards it end sud- 

 denly, sometimes in low cliff's, sometimes in steep slopes. The eastern 

 side of the valley is walled by the great uplift of Thousand Lake Mount- 

 ain. Along the western base of that mass runs one of the great faults of 

 the district, with a maximum throw of more than 4,000 feet. On every 



