362 



TRANSACTIONS OF THE CANADIAN INSTITUTE. 



[Vol. v. 



Another class of valleys may be seen between Vera Cruz and the 

 edge of the table-land, by following the route of the Mexican railway 

 from the coastal plain, just below Atoyac, to Esperanza, a town situated 

 upon the margin of the plateau, at an elevation of 8,000 feet above the 

 sea. The section is shown in figure 5, page 365, or on a scale less 

 vertically exaggerated in figure i. The valley may be considered as an 



Figure i. — Section between Esperanza and Atoyac, showing the descent of the valley by gradation steps, 

 or a succession of base levels of erosion. 



abrupt incision in the floor of the plateau. It is about forty miles long, 

 although the railway has a much greater length on account of its wind- 

 ings. The upper four miles, forming the amphitheatre heading the 

 valley, have a mean descent of about 600 feet per mile (although the 

 uppermost mile represents a descent of a thousand feet). If reaches 

 of eight or ten miles be taken, the mean slope is 150 feet per mile, 

 except below the city of Orizaba, where the declivity is about half as 

 great. The analysis of the slopes shows that they consist of very gently 

 declining, or almost level, steps, with abrupt frontal margins. Often 

 several steps coalesce so that, in places, they form one, several hundred 

 feet in height. In such cases, the platforms are dissected by short 

 canyons, such as may be seen at Atoyac, near Fortin, below Maltrata, 

 and at other places. The canyons characterizing the edges of the steps, 

 or terraces, are narrow and deep, and they are less than half a mile long, 

 representing the small amount of work since the last elevation of the 

 late base level of erosion. 



The more or less buried valleys crossing the coastal plains of the 

 southeastern part of the continent are frequently from two to four 

 miles wide, at distances of 100 or 200 miles from their mouths, and from 

 six to ten miles wide upon nearing the coast, which varies from fifteen 

 to nearly 300 miles from the edge of the continental shelf It has been 

 already stated that the Mississippi valley is from forty to eighty miles 

 \i{ide, and that the upper terrace plain of the Grand canyon of the 

 Colorado is from five to twelve miles wide, with bounding escarpments 

 2,000 feet high. The St. Lawrence is a partly drowned valley, seventy 

 miles wide for the last hundred miles of its course, before reaching 

 Anticosti Island, and about thirty miles in breadth for the next hundred 



