Excerpt from Geology and Mineral Resources of Santa Ysabel Quadrangle, San Diego County, California
The Santa Ysabel quadrangle lies along the east edge of the composite Peninsular Ranges batholith of Creta ecoas age. The oldest rocks are Triassic '2) schists which occur as s... mall roof pendants and as one large body in the vicinity of Julian. Jurassic gneissose quartz dio rite is intimately mixed with much of the schist, the two making a complex into which were intruded various members of the Cretaceous batholith. These intrusions are limited to gabbro and several tonalites. Structural features of the igneous rocks suggest that emplacement may have been accomplished by stoping in some places and by forceful injection in others.
Pleistocene continental gravels occur as remnants of a once continuous extensive blanket in the northern portion of the quadrangle.
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