Recognizing VOLCANIC ROCKS for the gold …
These are the commonest, and may be said to be the prevailing eruptive rocks associated with our ore deposits in Colorado, as for instance at Leadville, felsite porphyries as well as quartz porphyries occur in the granite rocks in the Central and Georgetown mining districts. All these rocks are common through the West, and quartz porphyries are the most common eruptive rocks the prospector is ...