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Show 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 Markagunt or the southern terraces overlooked by the Pink Cliffs. 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. KABBIT VALLEY. The slopes of the Awapa all converge towards a central depression called Rabbit Valley. The trachytic beds descending towards it end suddenly, sometimes in low cliffs, sometimes in steep slopes. The eastern side of the valley is walled by the great uplift of Thousand Lake Mountain. 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 |
