382 Tertiary Formations. paet ii. 



glassy feldspar, where, also, it is often amygdaloidal ; 

 it is never highly vesicular, except on the sides of rents 

 and on the upper and lower, spherically laminated sur- 

 faces. It is often columnar ; and in one place I saw 

 magnificent columns, each face twelve feet in width, 

 with their interstices filled up with calcareous tuff. 

 The streams rest conformably on the white sedimentary 

 beds, but I nowhere saw the actual junction ; nor did I 

 anywhere see the white beds actually superimposed on 

 the lava ; but some way up the valley, at the foot of 

 the uppermost escarpments, they must be thus super- 

 imposed. Moreover, at the lowest point down the 

 valley, where the streams thin out and terminate in 

 irregular projections, the spaces or intervals between 

 these projections are filled up to the level of the now 

 denuded and gravel-capped surfaces of the plains, with 

 the white-zoned sedimentary beds ; proving that this 

 matter continued to be deposited after the streams had 

 flowed. Hence we may conclude that the basalt is 

 contemporaneous with the upper parts, of the great 

 tertiary formation. 



The lava where first met with is 130 feet in thick- 

 ness : it there consists of two, three, or perhaps more 

 streams, divided from each other by vesicular spheroids, 

 like those on the surface. From the streams having, 

 as it appears, extended to different distances, the terminal 

 points are of unequal heights. Generally the surface 

 of the basalt is smooth ; but in one part high up the 

 valley it was so uneven and hummocky, that until I 

 afterwards saw the streams extending continuously on 

 both sides of the valley up to a height of about 3,000 

 feet close to the Cordillera, I thought that the craters of 

 eruption were probably close at hand. This hummocky 

 surface I believe to have been caused by the crossing 

 and heaping up of different streams. In one place, 



