This collection has been available for some time, but recently they have changed the appearance and made it a little easier to use. The website and all of the material is also reachable through American Memory (Library of Congress).
At Mudcat, in 'Links,' Music for the Nation (American Memory) is linked under 'Sheet Music', but for some reason the Brown Collection has not been linked directly.

The very useful Western Trails sheet music collection at the University of Colorado has not been linked at Mudcat. http://ucblibraries.colorado.edu/cgi-bin/sheetmusic.pl?WesternTrails&titlepages& 1
Western Trails