550 PHYSICAL GEOGEAPHY OF Book III. 
CHAPTEE VIII. 
Physical Geography of the North American Continent — Eetrospect of its 
Orologic Kelations — Southern Termination of the Kocky Mountains on the 
Upper Kio Grande — Their Southern Equivalents — Sierra Madre — Fre- 
quency of the name — The Great Sierra Madre of Cinaloa and Sonora — 
Its Northern Equivalent in the Californian Mountain-system — Interior 
longitudinal Basin of the "Western Half of the Continent — Depression of 
the Plateau between the middle Rio Grande and the middle Gila. 
Before I offer to my readers my observations and ex- 
perience during my residence in California, this will be the 
proper place to take a retrospect of a few of the great 
physico-geographical relations of the North American 
continent, respecting which I have been led to differ in 
some points from the prevalent opinions. In this I refer to 
the general arrangement of the mountains from the system 
of the Rocky Mountains west to the shores of the Pacific. 
I published a little treatise on the subject in a San Fran- 
cisco journal during my residence at that place, which 
has been reprinted in the annual scientific report of the 
Smithsonian Institute. A meritorious American geologist 
has since propounded opinions on the orology of America 
which differ from those I have expressed. This led me to 
reconsider the subject, but I still think I am correct in the 
following remarks. 
It is well known, that the erroneous idea that mountain- 
chains must necessarily be watersheds, and watersheds 
must be mountain-chains, has long prevailed among 
geographers, and especially map-makers, from their im- 
perfect actual knowledge of the earth's surface. Accurate 
topographical surveys, however, have not only corrected 
