Of the 3 main classes of rock, this one accounts for about 75% of all the exposed rocks on land
Archive - Show #3775, aired 2001-01-19 [PAR] Ah-one & ah-two, it's the marine snail in the photo looking at you ... Of the 3 main 
classes of rock, this one accounts for about 75% of all the exposed rocks on... [DOC] [TLE] The 3 basic rock types - U. of Oregon [PAR] What are the 3 basic types of rocks?