Element abundance on earth's crust

[Godwin Kessy]

May anyone help me on how to define the units used to define the abundance of elements on Earth's crust "It's written ppb(parts per billion by weight)"
I really fail to define that!

 

[chemisttree]

One ppb is one pound per billion pounds or one gram per billion grams. There are no units since they cancel.

ie. [STRIKE]pounds[/STRIKE] of silicon/billion [STRIKE]pounds[/STRIKE] crust = ppb silicon in Earth's crust