Note 1233 — Chapter XX · THE WORSHIP OF THE OAK
Reading edition
The only positive evidence, so far as I know, that the Celtic oak-god was also a deity of thunder and rain is his identification with Zeus (see above, p. 362). But the analogy of the Greeks, Italians, Teutons, Slavs, and Lithuanians may be allowed to supply the lack of more definite testimony.