198 FIELD COLUMBIAN MusEUM—-GEOLOGY, VoL. II. 
Weed gives in admirable detail a section about one hundred 
and fifty miles farthér north, his route leading from Parral in Chi- 
huahua via Guadalupe y Calvo to the Gulf of California.* Many of 
his general observations would apply to the region under discussion. 
The City of Durango, at which commenced the present writer’s 
journey over the region especially to be described, is situated upon an 
alluvial plain hemmed in by low but rugged hills. Eighteen miles to the 
southeast is a district known as La Brefia, which is quite celebrated 
for its volcanic features. Numerous caverns, extinct craters, crev- 
ices full of lava, and heaps of scoria are there said to abound. 
The area immediately about Durango is, however, compara- 
tively level. Its climate is semi-arid, and it is therefore treeless. 
Sufficient grass grows upon the plains, however, to permit of the 
raising of cattle and sheep on extensive ranches. The soil is fine- 
grained, and of a yellow color for the most part. About a mile to 
the north of Durango rises the remarkable Cerro Mercado, or Iron 
Mountain, a detailed description of which is deferred to a later page. 
The suburbs of Durango are drained by the Rio Tunal, which joins 
the Rio Mezquital and empties into the Pacific. The longitudinal 
valley to the north of Durango, however, is drained by streams which 
unite from north and south courses to flow eastward. From these 
streams results the Rio Nazas, which courses lazily through the broad 
basin known as the Desert or Bolson of Mapimi, until it reaches the 
Laguna de Mayran. This lake has no outlet. There is little doubt 
that at one time, however, a stream extended from it to the Rio 
Grande, two hundred miles to the north, for although the matter 
has not been studied from the topographic standpoint, a recent inves- 
tigation of the fish fauna of the Rio Nazas by Meekt shows that Rio 
Grande species of fish inhabit the Nazasin large numbers. The inter- 
ruption of the connection may be accounted for in three ways. 1. 
A warping of the valley. That some warping has occurred is indi- 
cated by the basin-like character of the region, also by the existence — 
of lakes in which the rivers terminate, showing drainage from more 
than one direction. No further evidence in detail regarding crust 
movements here is, however, at hand. 2. Increasing aridity of cli- 
mate. That such a change is taking place is commonly believed by 
residents of the region, who point, in support of their view, to areas 
now dry which they remember as once covered with water. Hewett 
is also inclined to favor this view. t 
*Trans. Am» Inst. Min. Eng., Vol. XXXII., pp. 444- 458. 
tAmer. Naturalist, Vol. XX XVII. se 778 
tAdvance sheet, Discussion of W. Weed’s Paper, Section Across the 
Sierra Madre Occidental of Mexico, nee Am. Inst. Min. Eng., Vol. XX XIII. 
