Josh has been living on the US border near the Rio Grande his entire life he has in checking one area of the river for about five years now he has noticed that it flows towards Mexico is that the United States what is Josh witnessing

You have been tasked with sorting the rock samples shown into three categories, with two rocks in each category. what attribute would you use to sort these rocks? which rocks would be in each category? rocks: obsidian, coquina limestone, granite, soapstone,pumice, and gneiss.