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)
The actual distance will be 12 km.
<u>Explanation</u>:
The scale of the map is 1:250000
1 cm in the map represents 250000 cm.
250000cm=2500 m=2.5 km
the measurement in the map is 4.8 cm
since 1 cm represents 250000 cm or 2.5 km
The actual distance represented by 4.8 cm on the map = 4.8*250000 cm =1,200,000 cm=12000 m=12 km
