The "Capped Die" 1879-CC Dollar earned its name from the crusty die rust that shows up as raised dimples around most of both letters of the mintmark. The Capped Die is also an overmintmark variety, though the rust obscures most of the details. Many collectors choose to obtain examples of both the normal (or "clear") mintmark and the Capped Die mintmark, so considerable demand exists for a date that is already scarce to begin with.
Of the two varieties, the Clear Mintmark is the more common of the two varieties by almost double. The Capped Die Dollar is very scarce as a Prooflike and downright rare as a Deep Mirror Prooflike. The finest examples certified by PCGS include 10 MS65s, 25 MS64PLs, and 4 MS64DMPLs.