How can I open *.ext files ?

Iras/c is a raster viewing application, but the .ext seems unknown.

Perhaps if we knew the source of the maps, as they could be a proprietary format?

Or try this, on the off chance?
http://mcmcweb.er.usgs.gov/drc/dlgv32pro/

Where did you get the map ".ext" files from?

It appears that all you have is a subset of data values that work with other data to produce the maps, e.g. you do not have the maps themselves.

Google says:

"Textural, statistical, geochemical, geophysical, dominant component, and color information are held in three separate, but similar, data files, based on the type of data: EXT, PRS, CLC."

Have you considered that these data files may also be copyright, or subject to license?