Answer:
Math
Explanation:
Sadly this is something to ask the math category. I'm sorry, I know this isnt helpful.
The quartz is more resistant to abrasion and chemically more resistant than the feldspar. The result is that the quartz remains in the stream but the feldspar breaks down to clay or is dissolved.
(this is from coursehero.com)
Abrasion is caused by the effects of wind and sand.