42 



MEXICO, CENTRAL AMEEICA, WEST INDIES. 



of Cliapala seem to imply that its basin was formerly far more extensive that at 

 present ; at that time it appears to have discharged its overflow westwards through 

 the valley of the river now flowing towards the Bay of Banderas, and some 

 engineers have proposed to cut a canal through this old fluvial bed. At the point 

 where the outlet was situated lava streams descended from the neighbouring 

 heights in prehistoric times. The issue was thus obstructed, and the waters were 

 forced to expand into a lake or else considerably to raise their level, and after- 

 wards seek a new issue through the lowest breach in the encircling hills. 



Fig. 20. — Lake Chapala. 

 Scale 1 : 1,500,000. 



,j'20' 





^-K 



West op Greenw cVi 



I02°20' 



30 Miles. 



These hills are in fact traversed by the Lerma through a series of gorges exca- 

 vated by erosion in the eruptive rocks. To judge from the extreme irregularity 

 of its course, this fluvial valley would appear to be of comparatively recent 

 geological date. Its whole bed is disposed like a gigantic flight of irregular steps, 

 where the stream develops a continued succession of high cascades and rapids, all 

 the way to the vicinit}^ of the coast. These gorges begin with one of the finest 

 cataracts in Mexico, named Juanacatlan from a neighbouring village. Rushing 

 over a precipice 65 feet high, the current acquires a tremendous impetus estimated 

 at 30,000 horse-power, and it is feared that the neighbourhood of Guadalajara may 

 tempt speculators to convert the falls into a series of reservoirs and mill races. 



Despite its abundant discharge, estimated at 4,000 cubic feet per second, the 

 Lerma is not navigable, and its bed may in many places be easily forded. But its 

 numerous ravines are scarcelj' anywhere accessible to wheeled trafiic or even pedes- 

 trians ; hence roads and tracks have had to be laid down across the escarpments of 

 the surrounding mountains. 



At Santiago, where the Rio Grande at last emerges on the low-lying coastlands, 

 it is still 145 feet above sea-level ; it enters the Pacific through a ramifjàng 



