They sacrifice upon the tops of the mountains
The highest part of them, nearest to the heavens, where they built their altars to idols, and offered sacrifice unto them, as we often read in Scripture they did: and burn incense upon the hills;
to their idols, which was one kind of sacrifice put for all others: under oaks, and poplars, and elms;
and indeed under every green tree that grew upon them, where there were groves of them raised up for this purpose; see ( Jeremiah 2:20 ) ( 3:6 ) : because the shadow, thereof is good;
the shadow of these trees, of each of them, was large, and preserved them from the sultry heat of the sun, as well as hid them from the sight of men; they could perform their idolatrous rites, as well as gratify their impure lusts, with more privacy and secrecy; and perhaps they thought the gods delighted in such shady places, and that these were frequented by spirits, and the departed souls of men; in such places the Heathens, whom the Jews imitated, built their temples, and offered their sacrifices F7. The "oak" is a very spreading tree; its branches are large, and its shadow very great: hence the religious Heathens in ancient times used to live under them, and worship them as gods, and dedicate temples to them, because they furnished them with acorns for food, and a shelter from the rain, and other inclemencies of the heavens F8; particularly the oak was consecrated to Jupiter, as appears from what Virgil says F9. The oak at Dodona is famous for its antiquity, where were a fountain and groves, and a temple dedicated to the same Heathen deity; and from whence oracles were given forth F11. The Druids here in Britain chose to have their groves of oaks; nor did they perform any of their sacred rites without the leaves of them: hence Pliny F12 says they had their name. The "poplar" mentioned is the white poplar, as the word used signifies, and which affords a very hospitable shadow, as the poet F13 calls it; and this was a tree also with the Heathens sacred to their gods, particularly to Hercules F14; because it is said he brought it first into Greece from the river Acheron, where it grew; and the wood of no other tree would the Eleans use, in preparing the sacrifices for Jupiter Olympius F15. The "elm" is also a very shady tree; hence Virgil