Complex sentence: About 1900, in search of quicksilver, two tunnels were driven into the canyon side a short distance downriver from the springs, and although the rock contains noticeable amounts of cinnabar, but this mineral was not found in paying quantity.
Split into simpler sentences: About 1900, in search of quicksilver, two tunnels were driven into the canyon side a short distance downriver from the springs. The rock contains noticeable amounts of cinnabar, but this mineral was not found in paying quantity.