Amountain stream drains an area that is predominately a coarse grained granite. the granite has the basic composition of 15% quartz and 85% feldspars. however, a few miles from the mountains, an inspection of the stream sediments reveal that the composition of the sand is transporting is about 75% quartz and only 25% feldspars. why is there such a difference between the source of the sand (granite) and what is found in the stream that drains the granite mountains?