  We prove the Mixing Conjecture of Michel--Venkatesh for the class group
action on Heegner points of large discriminant on compact arithmetic surfaces
attached to maximal orders in rational quaternion algebras. The proof is
conditional on the Generalized Riemann Hypothesis, and when the division
algebra is indefinite we furthermore assume the Ramanujan conjecture. Our
methods, which provide an effective rate, are based on the spectral theory of
automorphic forms and their $L$-functions, as well as techniques in classical
analytic number theory.
