dacite - large hand/display specimen of gray porphyritic dacite, a rock common in the Archean continents These specimens are moderately cemented

These specimens are moderately cemented and will stand up to student examination when handled carefully

The rhombohedral cleavage is less important than the effervescence in dilute hydrochloric acid

The brown in this calcite is entirely natural

Two views of the same specimen

probable source of this sand is the Big Sur River

dacite - large hand/display specimen of gray porphyritic dacite, a rock common in the Archean continents These specimens are moderately cementedDacite was a common rock in the Archean continents, forming when hot young oceanic crust subducted beneath thick continental plates. As the hot oceanic plate subducted into the mantle, it partially melted, dewatered and became hydothermally altered, with the melt gaining quartz and sodium and forming a magma that differentiated and became even more sodium and silica rich as it rose. As it cooled near the surface, quartz (sodium silicate), plagioclase