| A porphyry is an igneous rock that contains some large
crystals floating in a fine-grained matrix. This is really a textural term and the rock is
usually named for its overall composition, which may vary from felsite through andesite to basalt. However, this
composition may not always be determinable without laboratory investigation, particulary
for felsic to intermediate composition rocks. Consequently, the rock may be called a
"quartz porphyry", "feldspar porphyry", etc., depending upon the
nature of the phenocrysts (the large crystals) present. |
