(River Megaliths isn't a spell.)
River Megaliths enters the battlefield tapped.
, : You may put a land card from your hand onto the battlefield.
: Add or to your mana pool.
Put River Megaliths on top of your library: Add to your mana pool.
, 
: You may put a land card from your hand onto the battlefield.: Add 
or 
to your mana pool. to your mana pool. 
