In greenstone, the olivine and peridotite that made up the fresh basalt have been metamorphosed by high pressure and warm fluids into green minerals—epidote, actinolite or chlorite, depending on the exact conditions. The white mineral is aragonite, an alternative crystal form of calcium carbonate (its other form is calcite).
Rock of this kind is manufactured in subduction zones and is seldom brought to the surface unchanged. The dynamics of the Californian coastal region make it one such place. Greenstone belts are very common in Earth's oldest rocks, of Archean age. Exactly what they mean is still not settled, but they may not represent the kind of crustal rocks that we know today.
For more photos see the Metamorphic Rocks Gallery.
Other galleries:
Fossils
Geologic Features and Processes
Glaciers and Ice
Landforms
Minerals
Rocks
Geology and Society