  The leaves in singular holomorphic foliation theory are examples of
quasi-analytic layers. In the first part of our publication we are concerned
with a theory of these subjects. A quasi-analytic decomposition of a complex
manifold is a decomposition into pairwise disjoint connected quasi-analytic
layers. These are holomorphic foliations in the sense of P. Stefan and K.
Spallek. A very different but more usual conception of holomorphic foliations
is develloped by P. Baum and R. Bott. It is based on holomorphic sheaf theory.
In the second part we study the relation between quasi-analytic decompositions
and singular holomorphic foliations in the sense of Baum and Bott.
