198 GEOLOGY OF THE HIGH PLATEAUS. 



cluster of rolling hills, rarely exceeding 300 feet in altitude above the 

 platform on which they stand, covered with soil mingled with decayed 

 vesicular cinders. Their true nature is disclosed by the scoriaceous char- 

 acter of the fragments which constitute the greater portion of their mass. 

 It will be remembered that basaltic craters, when well preserved, are rather' 

 symmetrical truncated cones, with conical or funnel-shaped depressions at 

 the summit, and the entire mass is composed of vesicular fragments blown 

 out by the escaping steam and gases and falling with approximate uniformity 

 around the orifice. The spongy character of these fragments renders them 

 an easy prey to the chemical forces of the atmosphere, and they are readily 

 decomposed. After thousands of years of weathering these cones are 

 literally dissolved, losing their lime, iron, and alkali, while the alumina and 

 silica remain, and the cone gradually loses its form and is reduced to a 

 shapeless heap of soil with commingled cinders in every stage of decay. 

 Around the bases of these ancient cones we find half-revealed sheets of 

 basaltic lava. Any eruption may be followed by the building of a cinder- 

 cone, and most basaltic outbreaks are so supplemented (at least in this dis- 

 trict) ; but it is not always so. A considerable number of the basaltic sheets 

 have been disgorged where no trace of a cone remains, and some of these 

 are so recent that the last thousand years may have witnessed the catas- 

 trophes.* It is notable that the most extensive outpours are most frequently 

 without them. Among the basalts of the locality of which we are speaking 

 are many cinder-cones in an advanced stage of decay. The floods of basalt 

 which have emanated from them lie in many sheets, none of which indi- 

 vidually present great thickness, but by superposition have built up this 

 part of the plateau from 500 to 800 feet above the normal platform. They 

 are for the most part concealed by their own ruins, but numerous ravines 

 have been cut into them, showing in many places their edges and giving a 

 general idea of their mass and distribution. They rest upon older trachytes 

 and occasionally andesites which had been scored by ravines before the 

 basaltic outbreaks, and in a number of places the uneven surfaces of contact 

 are clearly revealed. 



* I am spcaliiiig in general terms of the basalts. Tliosc of the locality just spoken of aro all 

 probably older than the Quaternary. 



