Many of these are thousands of feet in depth; and the road that enables me to reach their bottoms is often no more than a narrow ledge of the impending cliff, running terrace-like over a foaming torrent.
Still onward and upward I go, until the "foot-hills" are passed, and I enter a defile of the mountains themselves--a pass of the Mexican Andes.
I ride through, under the shadow of dark forests and rocks of blue porphyry.

I emerge upon the other side of the sierra.

A new scene opens before my eyes--a scene of such soft loveliness that I suddenly rein up my horse, and gaze upon it with mingled feelings of admiration and astonishment.

I am looking upon one of the "valles" of Mexico, those great table-plains that lie within the Cordilleras of the Andes, thousands of feet above ocean-level, and, along with these mountains, stretching from the tropic almost to the shores of the Arctic Sea.
The plain before me is level, as though its surface were liquid.

I see mountains bounding it on all sides; but there are passes through them that lead into other plains (_valus_).