232 GEOLOGY OP 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 correspondence — 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 table — at least, none has been observed — 

 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 arrange- 

 ment 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 extruvasated matter, that it at once spread out in 

 wide fields, and deluged the surrounding country like a tide in a bay flow- 

 ing over all inequalities. How far these floods extended it is difficult to 



