352 TRANS. OF THE ACAD. OF SCIENCE. 
which, as we have already seen, has been, and that, too, atno 
very remote geological period, the theatre of intense igne : 
action. Me a exception of the portion constituting the 
here ro 
and ‘the m si aga well as could be determined ata ie 9 
ance of thirty or forty miles, present everywhere precipit- 
auly once = east. Between these and the Rio Grande, 
the sur xceedingly rough and broken, being thi 
marked with ax and low conical hills, most of which ap 
pear to be composed of unstratified rocks. 
. ving concluded the examination of the Fra Ciistoral : 
- Mountain, we again started for camp, which, according to ae 
valley, and which we reached : at a _ hour 
greatly fatigued. 
Rio Gra nde is about 
