552 SOUTHEEN TERMINATION OF Book III. 
the road down into the valley of the Eio Grande leads, 
on almost level ground, through a narrow passage in 
the rocks. Supposing that the sandstone ridge through 
which this pass runs is to be considered a part of the 
Rocky Mountains, this ridge will in that case lie on the 
eastern side not only of the Rio Grande but of the Rio 
Pecos, its southern continuation terminating in the plateau 
of Western Texas. But between the Upper Pecos and 
the Rio Grande, the Santa Fe road is bounded on its 
north side by high Alpine mountains, covered with snow 
during the greater part of the year. Southwards continues 
a range of isolated mountain groups, to which belong the 
Placer, Sandilla, and Manzana mountains. The road runs 
round the extreme southern promontories of the higher 
chain over table-lands six to seven thousand feet above 
the level of the sea, which terminate abruptly towards the 
valley of the Rio Grande, and the declivities and detached 
portions of which present here a system of mountains 
quite of a different geological character to that of the 
isolated groups rising above the edge of the plateau, — the 
rocks of which are of a plutonic and metamorphic nature. 
Regarding these mountain groups and table-lands as the 
southern continuation of the Rocky Mountains, these are 
also on the eastern side of the Rio Grande, and likewise 
terminate in the Texan plateau. In one point of view, it 
is correct to recognize in these mountains really a conti- 
nuation of the Rocky Mountains ; but it must be clearly 
understood that it agrees better with the general orological 
system of this country to connect them with that range 
which commences on the western side of the Upper Rio 
Grande, and the subordinate branches of which succes- 
sively cross the river in the space between Santa Fe' and 
the bend of the river near Santa Barbara. 
