When salty ocean water evaporates the salt forms cubic shaped crystals.
A quartz rich solution has coated and replaced the salt (halite) crystals
with quartz (silicon dioxide)
So the crystal shape is that of salt but the crystals themselves are
composed of quartz.
This is called a "pseudomorph" :Pseudo" meaning false and "morph" meaning
shape. The quartz is in the "false shape" of salt.