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New Mexico State University: Metamorphism and Metamorphic Rocks [Pdf]
A PowerPoint slideshow converted to a PDF, with six slides per page. It explains what metamorphism is, the distribution of metamorphic rocks, the agents of metamorphism, the types of metamorphism, metamorphic grades or degrees, how...
US Geological Survey
U.s. Geological Survey: Limestone and Marble
An explanation of the differences between limestone and marble, including some excellent pictures.
Australian Museum
Australian Museum: Metamorphic Rocks
Learn the process by which metamorphic rocks are created, where they are located, and how they are identified by texture (foliated and non-foliated) and mineral composition.