Subject: How does realtive dating work?

In my Biology class at my high school we are learning about evolution and I 
got confused about relative dating and when I asked my teacher she didn't know 
the answer so I though you might. My textbook is a 7th edition Prentice Hall 
Biology book. On page 575 it says "If these sedimentary layers contain 
fossils, scientists use the layers to determine when certain organisms existed 
in relation to others." It looks to me like they are saying that the layers 
date the fossils. But, on page 578 it talks about the trilobite being a good 
index fossil and it says, "Some species of these shelled marine animals only 
lived between 500-600 million years ago. Scientists know, therefore, that 
sedimentary rock layers containing trilobite fossils must be between 500-600 
years old." On this page it looks like the fossils are used to date the 
layers. So I guess my question is, how can fossils date the layers when layers 
date the fossils? Thanks for your time. Stephanie

Re: How does realtive dating work?