208 FIELD COLUMBIAN MuUsSEUM—GEOLOGY, VOL. II. 
a considerable distance eastward, a comparatively abundant rainfall 
takes place. The case seems somewhat analogous to one cited by 
Dana from Oahu,* where chilling of the moisture-laden winds is ac- 
complished by a cliff so steep that precipitation does not take place 
until the winds have passed beyond the summit, and the rainfall ac- 
cordingly takes place almost wholly upon the leeward slope. 
Ventanas is a small mining-camp of about six hundred inhab- 
itants, The name Ventanas, meaning ‘‘the windows,” refers to two 
large rounded orifices to be seen piercing a mountain wall some dis- 
tance south of the town. By this name the town is commonly known, 
although its official designation is Villa Corona. It is situated by 
the side of the Rio del Presidio, sometimes known as the Mazatlan, 
a swift, wide stream, fordable in the dry season but in the rainy 
season rising to such a height that supplhes must be drawn across 
upon a wire cable stretched for the purpose. The elevation is about 
2,000 feet above the sea level, and the situation about equidistant 
between Durango and Mazatlan. 
The principal mines in the vicinity have been operated for a 
number of-years by the Ventanas Mining and Milling Company, 
an American corporation. The ores are chiefly antimonial sulphu- 
rets of silver, stephanite being the most abundant and character- 
istic silver-bearing mineral. Only ores of high grade are mined, 
it being impossible under such disadvantages of transportation to 
work low-grade ores at a profit. Some of the richest ores are stated 
to carry a value as high as 500 ounces silver, and 10 ounces gold to 
the ton. The most important mines are situated at a level of about 
3,500 feet. Workings near the summit of the peaks, although often 
promising good returns, have not proved remunerative. This ac- 
cords with Weed’s observation,t that the profitable mines are in the 
underlying andesitic rocks. At La Concepcion mine the ore occurs 
in a well-defined vein 8 to 10 feet wide. The gangue is a tough, 
hard, white quartz, in which stephanite and a considerable amount 
of chalcopyrite and pyrite are distributed. The rock adjoining 
the vein is of a highly siliceous character, resembling a quartzite. 
Dotted throughout it, however, occur flecks of other metamor- 
phosed minerals, indicating that the rock is in reality an andesite 
highly impregnated with silica. The mining is done by hand and 
the ore packed on burros to the mill located in the village. The 
river here furnishes both power and water for leaching the ores. 

*Manual of Geology, 1895, p. 179. 
TOP. ctt., p. 446. 
