106 



tended like a map beneath the giants of the mesa; and were it not f ortheir 

 steep and intricate forms, the windings of the river valleys could be easily 

 traced through the transparent atmosphere. The clearness of the air in 

 these regions is remarkable, even for the Pacific coast; the stars at night 

 shine with astonishing brilliancy; and a lofty mountain of striking form, 

 the Cerro de Alamos, is visible all day at distances over 60 miles, where 

 nothing intervenes, except in the rainy season. To the south-east of Pan- 

 dura is the peak of Metate standing on a spur of the mesa, isolated by the 

 erosion of two streams, and surrounded on three sides by immense verti- 

 cal precipices. There is a wide difference between the shapes of mountains 

 in this latitude, and the ice-planed forms of Europe; as long as the rivers 

 can deepen their beds they do not widen the valleys at all. which remain 

 either canons enclosed by cliffs, or Y-shaped cuttings, usually terminating 

 at the bottom in a narrow gorge. On the lower ridges the evergreen flora 

 is more varied and abundant; three varieties of oak, three of pine, and one 

 of arbutus are common; the arbutus, whit its thick stem, bright -orange 

 red bark, dark green leaves, and white flowers, is very picturesque; and 

 the Encina roble, an evergreen oak, with thick leathery leaves, is very dis- 

 tinct. Lower down, below 5000 feet, agaves, mamillaria, and prickly pears 

 (Opuntia) are abundant among the grass that covers old geological for- 

 mations, or under the dwarf forest that flourishes on recent volcanic ejec- 

 tamenta. Following the rigde towards Guaza, the track overlooks a wide 

 basin, grass-covered and dotted with dwarf oaks, into which two streams 

 converge at an acute angle, leaving between them a thin slice of table- 

 land, standing up perpendicularly 3000 feet above the stream, and revea- 

 ling its structure of horizontal beds; the upper pale acid lava, and the lo- 

 wer of dark basic materials. Such wedges are common at the margin of 

 the mesa, and are sometimes isolated into towers that eventually crumble 

 into conical forms; when first isolated, they are locally called cahallos (hor- 

 ses); the ridges produced by erosion often terminate in a diminishing row 

 of peaks that have been formed in this way. This flamboyant style of 

 mountain sculpture shows that no severe earthquakes have affected this 

 part of Mexico for a long time, as many of the grotesque rock-pillars and 

 lofty partition walls have such slender bases that a violent shock would 

 certainly overturn them. 



At Guaza theChinipas and Septentrion rivers join, and there is a lit- 

 tle level ground laid down by the rivers during a temporary obstruction 

 of the lower gorge, caused by recent volcanic eruptions, to be further no- 

 ticed. Betwen Guaza and La Junta, where the main river forms, the va- 

 lley has been eroded to the base of the volcanic series, exposing syenite; 

 a calcerous fossil was also observed in the shingle, though time did not 

 allow a search for its parent stratum. 



