Of the 3 main classes of rock, this one accounts for about 75% of all the exposed rocks on land
... Geologists distinguish three main groups of rocks (with of course 
numerous .... because about 75% of the earth's exposed land surface consist of 
sediments... [DOC] [TLE] Chapter 4 Rocks and Minerals - Volcano World [PAR] Chapter 4 focuses on Rocks and Minerals, looking at types of rocks.