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Show 232 GEOLOGY OF THE HIGH PLATEAUS. the name basalt nearly, if not quite, the whole category of dark-gray and black augitic rocks of rather fine-grained texture, high specific gravity, and more or less conchoidal fracture. To the range of variation which is now known to extend through this class both in respect to chemical and min-eralogical constitution he appears to have attached little importance, and, indeed, was unacquainted with such distinctions as have been established by later researches. It has seemed to me possible that the earlier rocks which he has called basalt may prove to be augitic andesite, while the most recent ones are the most basic of their class, and therefore identical with the rocks now assigned by more recent classification to basalt in the more restricted sense of the term, and finally that intermediate varieties may there exist, which are equivalent to those rocks which I have here designated as doler-ite. At all events, there is this correspondenceâ€"both localities present the intercalation of augitic-plagioclase rocks with trachytes. Let us now examine the east side of the plateau directly across from the great amphitheater. Another grand exposure is presented here. There is no fault on this side of the tableâ€"at least, none has been observedâ€" but a large valley has been excavated not perpendicularly inwards towards the axis of the plateau, but very obliquely, cutting off the gable-like end of Blue Mountain. This name is given to that high knob which stands upon the eastern verge of the plateau, at the end of the transverse ridge which now marks the locus of one of the centers or axes of eruption. The excavation of the valley has cut off the eastern face of this ridge and laid open the structure and arrangement of the various beds. This arrangement is quite similar to what would be expected and to what has often been observed in great volcanic piles. From the central axis the sheets are seen dipping away in both directions at variable angles never very great. On the northern side they descend towards the northeast and on the southern side to the southeast, the lower beds dipping more than the upper ones. All of these lavas seem to have welled up in mighty floods without any of that explosive violence which often characterizes volcanic action, and so great was the volume of extravasated matter, that it at once spread out in wide fields, and deluged the surrounding country like a tide in a bay flowing over all inequalities. How far these floods extended it is difficult to |