E103 Introduction to Rocks    This is an outstanding rock identification chart. The introduction explains that each type of rock is the result of a specific mixture of minerals subjected to a clearly defined geological process. It show and describes the most common rock-forming minerals and variations that occur with rock types.     The bulk of the poster is divided into three sections, one for each rock type: igneous, sedimentary and metamorphic. An illustration shows how they are made and a photograph shows a natural formation. A dozen other photographs show representative specimens. Each is a companied by an informative caption citing identifying characteristics and common variations.    A bonus section shows and explains some of the more specialized and popular rock forms, such as geodes, concretions, agates and meteorites. This is a great poster for earth science classes and is  especially useful to collectors.    . Laminated No. E103L $18.95 Non-Laminated No. E103F $9.95