108 



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

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

 traced through the trausparent atmospliere. 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 sonth-east of Pan- 

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

 erosión of two streams, and snrrounded 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 f orms of Europe; as long as the rivers 

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

 either cañons 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 rohle, an evergreen oak, -with thick leathery leaves, is very dis- 

 tinct. Lower down, below 5000 f eet, agaves, mamillaria, andpricklypears 

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

 mations. or under the dwarf f orest that ñourishes 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 perpendiculary 3000 feet above the stream, and revea- 

 ling its struoture of horizontal beds; the upper palé 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 f orms; when first isolated, they are locally called cahallos (hor- 

 ses); the ridges produced by erosión often termínate 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 aífected this 

 part of México 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 the Chinipas and Septentrión rivers join, and there is alit- 

 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- 

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

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

 allow a search for its parent stratum. 



