Electrical wiring 14-2 wire connect 12-2 wire
You could use the 12 awg wire on a 15 amp circuit connecting the section that is wired in 14.
 The only problem and I have found with this in the past where part was wired in 14 but the breaker had #12 , I am guessing that at a later date someone put in a 20a breaker and cooked the 14 gauge wireing because it looked like it was wired with 12 at the panel.