Section 3. The Tropics North of the Equator 



A. Mexico and Central America 



1. MEXICO 



By E. W. Nelson and E. A. Goldman 



I. Introduction 



The following brief outline is based on 

 information gathered by the authors in 

 the course of field investigations in 

 Mexico, begun in 1892, and continued 

 with short interruptions until 1906. 

 Nearly all of the higher mountain 

 ranges and principal valleys were vis- 

 ited, and field travel extended into 

 every state and territory in the Re- 

 public, including an overland journey 

 the entire length of Lower California. 



II. General Features and 

 Native Biota 



a. topography 



Mexico, extending from about latitude 

 14° to 32° 42' North, and longitude 86° 

 40' to 117° 10' West from Greenwich, 

 presents an extraordinary diversity of 

 physiographic and biologic conditions. 

 It is crossed near the center by the 

 Tropic of Cancer, well to the south of 

 which the land mass culminates on 

 Mount Orizaba (having an elevation of 

 more than 18,000 ft., and the highest 

 mountain in North America south of 

 Alaska) in an elevated section including 

 other timberline peaks, one of which, 

 the extinct Volcano of Ixtaccihuatl, 

 bears small receding glaciers on its 

 upper slopes. 



All of the 12 or 15 highest mountain 

 peaks of Mexico are of volcanic origin 

 and are grouped about the southern 

 border of the tableland. Orizaba, on 

 the east, fronts the Gulf of Mexico, and 

 the Volcano of Colima looks over the 

 Pacific. Scattered irregularly between 

 these peaks are the other higher moun- 

 tains, including those about the Valley 



of Mexico, The Volcano of Colima is 

 still active at intervals and Popocatepetl 

 sends out smoke and steam from small 

 vents in the crater walls. 



The salient physical features of the 

 Mexican mainland are simple. The 

 greater part of the area is occupied by 

 what is termed the tableland or great 

 interior plateau. This is an obtusely 

 triangular area having an elevation of 

 from 1500 to 3000 ft. in the north and 

 rising from 7000 to 8000 at the apex of 

 the triangle, which forms its southern 

 end, in the region of the Valley of 

 Mexico. The tableland has the form 

 of a rolling plain broken here and there 

 by escarpments and with irregularly 

 scattered mountains rising island-like 

 on its surface, some of them attaining 

 altitudes from 6000 to 11,000 ft. On 

 the west the tableland is bordered by 

 the Sierra Madre Occidental, as the 

 southern extension of the Rocky Moun- 

 tains from the United States is called, 

 and on the east by the Sierra Madre 

 Oriental. 



For most of the distance on the eastern 

 and western flanks of the tableland 

 these mountains slope up so gradually 

 from the tableland that they do not 

 appear as large ranges. On the eastern 

 and western fronts, facing the coasts, 

 the rise from the low base level on the 

 coastal plains is abrupt and often rug- 

 ged, presenting the contours of strongly 

 developed mountain ranges. Between 

 the low coastal bases of the ranges 

 extend gently sloping coastal plains 

 varying from 25 to more than 100 mi. 

 in width, across which flow the rivers 

 draining the mountain slopes. The 

 southern end of the tableland narrows 

 with the general narrowing of the con- 

 tinental area, which brings the mountain 

 ranges together and forms a broken 



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