HKDLicK.\] PHYSIOLOGICAL AND MEDICAL OBSERVATIONS 3 



The general surface geology of the tract appears to be quite simple. 

 In the northern part the formations are largely Jura-triassic, with 

 some volcanic features, while farther south the exposed rocks are 

 either of volcanic origin or are limestones or granites. The highest 

 plateaus are evidently remains of old extensive plains, the valleys, 

 canyons, and other depressions being products of erosion. At pres- 

 ent volcanic activity exists only in the extreme south of the area 

 dealt with in these studies, in Mexico, but hot springs are found 

 throughout the region. Earthquakes, while occurring occasionally, 

 are rarely destructive, except in the vicinity of the volcano of Colima, 

 in Mexico. 



The elevation of the areas inhabited by the Indians some or all of 

 the time varies from a little above the sea level along the Pacific 

 coast to about 8,000 feet in the interior. The large plateaus range 

 in altitude from 3,000 to 5,000 feet, the highest mesas reach about 

 10,000 feet above the sea, and a few of the peaks rise above tliis 

 elevation. The plains are settled only where there is a constant 

 water supply, and the liighest mesas are inhabited, if at all, only at 

 intervals. The climate of tliis territory, with a few exceptions, is 

 characterized by marked uniformity. From southern Utah and Col- 

 orado to the City of Mexico, except in the more southern coast area, 

 the period corresponding to the fall, winter, and spring of the middle 

 Atlantic seaboard constitutes the dry season. During tliis season 

 vegetation is dormant and animal life scarce. The air is very dry 

 and clear, and excessive evaporation takes place. The valleys and 

 the lowlands in general are hot in the daytime, the temperature in 

 the shade often exceeding 95° and 100° F., but the nights are invari- 

 ably cool. In the more elevated portions of the vast area under con- 

 sideration the temperature diu"ing the day in the dry season is always 

 bearable and often very pleasant, while the nights are cold. During 

 the winter months the temperature falls below the freezing point. 

 From January to March the dry spell is somewhat interrupted and 

 snow and rain fall at irregular intervals. Only rarely is the rain- 

 fall at this time heavy. Along the Pacific slope of Mexico, in south- 

 ern Sinaloa and Tepic, precipitation is more frequent and there is a 

 soi.u"ce of additional moistiue in the dews. In October, for example, 

 at the beginning of dry weather in the latter region, the writer's party 

 was greatly inconvenienced by unhealthful morning dews that were 

 quite equal to moderate showers. 



During the dry season there occur frequently on the sandy pla- 

 teaus, particularly those of New Mexico and Arizona, more or less 

 violent and protracted "sandstorms." These result, directly or 

 indirectly, in much physical suffering to the Indian, though only 

 exceptionally are they dangerous to life. They interfere also wdth 

 agriculture. 



