Carbon is not used to date geologic rock ages because

1 - no rocks contain carbon

2 - Radioactive carbon is constantly being created in the atmosphere

3 - Radioactive carbon decays too quickly in life

4 - we have used up all the coal

1

Explanation:

All organic matter contains carbon

however, rocks and minerals and such are inorganic, containing no carbon. If carbon is to be found in rocks then it is likely to be organisms (dead or alive) within those rocks and not the rocks itself.