Chap. VII. VOLCANIC HILLS IN THE VALLEY. 305 
top to another is treeless, and like a large basin in the 
desert. 
Of this general character I formed a clear idea in 
ascending a small isolated mountain, of volcanic origin, 
rising out of the alluvial masses of the eastern side- 
terrace. The lower masses of the rock consist of a 
dense, grey, trachytic lava; the upper, of a lava of 
darker colour and more basaltic or doleritic nature, be- 
coming more and more porous and spongy towards the 
summit, till, on the top, it appears in black lumps thrown 
one over another, perforated, eaten into, jagged, crooked, 
and forced up, like a scoria not completely vitrified. The 
rock contains, from top to bottom, numerous concretions 
of a milk- coloured chalcedony, sometimes hyalitic, small 
pieces of which lay strewn about everywhere. The view 
from the summit of this isolated mountain is grand, and 
most peculiar. Wherever the eye stretches to the edge 
of the wide prairie valley it sees in the distance some 
detached mountain-group, its bare and rugged sides de- 
scending to the valley, and from whose base the slanting 
plain of debris, clothed in a naked and monotonous grey- 
brown, descends in long lines to the river, which is indi- 
cated only by some distant poplars. 
I saw upon this summit the first shrub of the Larrea 
Mexicana, which, further south, forms such an important 
feature in the vegetation of the steppes, from the plateaus 
of Texas down to the lower Gila, and which in the 
valley of the Rio Grande likewise begins to predominate, 
only farther south. It is remarkable, in approaching the 
extensive province of certain plants, to see them first 
appear upon the summits of isolated mountains, and espe- 
cially when, as here, the district of this vegetation lies 
further south. This remark applies also to the Opuntia 
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