Conglomerate Cobbles at Granite Point

The dark purplish speckled rocks are an ancient volcanic rock, a devitrified, phenocryst-bearing welded tuff of Early Jurassic age from an unknown source. Burnham calls it the purple porphyry, and she has been looking from Mexico to Nevada for its source without success. There are also pieces of the granodiorite and a suite of about a dozen less common rock types. This blend of rock types is found at Point Reyes in nearly the same proportions. Compare the photos from Point Reyes to this assortment of clasts from Point Lobos.