jackhammer and a pick my friend. although you have to experienced in rock fissures to use the hammer. rock is a bitch to work and you really have to be determined to make your trail. hopefully it's already pretty rideable and you only have to smoothen out a few spots.
Granite is super slippery when it is wet. It is like riding on ice. I would love to see someone do a wall ride when it is raining on a granite rock. The only way to cut granite is with a diamond tooth saw blade and it will honestly take a month to cut a solid chunk of granite. The heat from the friction will also make you go through these expensive blades like you change your underwear. It is also really heavy and you can't move it easily. Find somewhere else to build.