Time to get close to the rocks. The lavas of the Buttes are so packed with crystals that they could barely flow. The larger whitish grains are crystals of sanidine, a high-temperature form of potassium feldspar, and the smaller black grains are hornblende and biotite. The groundmass can be either gray or red, depending on chemical conditions near the surface. Red rocks were altered at high temperature by steam (see an example inside a cinder cone). Andesite is a volcanic rock that is higher in silica content than basalt, which gives it a lighter color and a different set of minerals. A rock like this, with a population of large crystals (phenocrysts) in a fine-grained groundmass, is called a porphyry or, more correctly, a rock with porphyritic texture.
Hausback told our group that the porphyritic andesite of the Sutter Buttes is on display in the Smithsonian Institution as a museum-worthy example of the type. He also said that the rock was quarried for use lining canals, reinforcing riverbanks and bolstering levees all around the Central Valley. This photo shows the surface of one such boulder, hard at work by the side of West Butte Road near state route 20.
