Figure 5. Intrusive rocks on the 1985 Bedrock Geologic Map of Maine are described by age (chart to the left) and composition (chart to the right). Age is indicated by colors similar to those used for stratified rocks of the same age, but in this case various shades reflect the plutonic rock composition. A random slash pattern is embedded in the color to indicate a plutonic rock unit. Plutonic rock compositions are indicated by a number code from 1 to 10 which may be followed by a letter or letters, defined in the list shown here, and keyed to the compositional ranges shown on the triangular diagram. In using this diagram to classify a plutonic rock, its quartz percentage determines the height above the base of the triangle, and its alkali feldspar to plagioclase feldspar ratio determines the position from left to right. The presence of other important minerals or attributes is indicated by additional letters in parentheses.

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Last updated on March 7, 2008

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