The hornblendes of the San Marcos gabbro are divided into five types according to their color, habit, and occurrence. The pale-green hornblende of the olivine rocks and the deep-green hornblende of the norites are the earliest types and were formed largely by reaction with olivine and pyroxene. The coarse green-brown poikilitic hornblende is predominantly primary. The pale fibrous and blue-green hornblendes are late and of hydrothermal origin.

Four primary igneous structures of the San Marcos rocks are described under the terms auto-injection structure, banded structure, nodular structure, and orbicular structure. These structures were all formed at the close of the period of mobility of the magma, when it was filled with plagioclase crystals, and are related to local differences in mineralizer content, as shown by the distribution of hornblende, as well as to various features of the final movements of the mass.

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