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Research has shown igneous rock from the Devonian period ( around 400 million years ago ) intrudes into the surrounding metamorphic schists ; the intrusions take the form of a series of concentric ring dikes .
 Devonian
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Devonian