Abstract
Volcanic rocks of Miocene age belonging to the San Juan, Silverton, and Potosi series have been intruded by a stock which shows a sequence of intrusions from diorite to gabbro to diorite, and finally to rhyolite. The stock is excellently exposed for a vertical distance of 1900 feet. The first diorite intrusion was locally transformed after emplacement to a hornblende monzonite facies, probably by gaseous transfer. Many radiating andesitic dikes are related to the diorite. Gabbro, the next intrusion, has an associated quartz monzonite facies that was differentiated at depth. Many fragments and large masses of pre-Cambrian quartzite were carried up in the gabbroic magma. Clastic dikes unquestionably formed from material derived from lower-lying conglomeratic beds penetrate the gabbro. A uniform, fine-grained diorite with associated andesite and dacite dikes represents the third intrusion. The last intrusion is a rhyolite which probably was emplaced after a considerably greater time interval than existed between the diorites and gabbro. Two rocks from the stock were chemically analyzed, and numerous modal analyses are tabulated. The undisturbed, nearly horizontal attitude of surrounding volcanic rocks has interest in regard to the mode of emplacement of the stock.
