Analysis of As some vast Tropic tree, itself a wood (fragment)

As some vast Tropic tree, itself a wood,
   That crests its Head with clouds, beneath the flood
   Feeds its deep roots, and with the bulging flank
   Of its wide base controls the fronting bank,
   (By the slant current's pressure scoop'd away
   The fronting bank becomes a foam-piled bay)
   High in the Fork the uncouth Idol knits
   His channel'd Brows; low murmurs stir by fits
   And dark below the horrid Faquir sits;
  An Horror from its broad Head's branchy wreath
  Broods o'er the rude Idolatry beneath--