Volcanic islands fed by the partial melting of the Farallon Plate, due to high pressure and temperature at great depth, form just east of the deep ocean trench that marks the boundary between the North American Plate and the Farallon Plate. By this time period, the great thickness of Continental Sediments and volcanic rocks deposited upon the North American Plate are compressed into a great mountain range similar to the Andes in South America. The cold massive basaltic slab of the Farallon Plate continues to slide underneath the North American Plate. Giant magma bodies melt up into the overlying crumpled Jurassic age sediments, destroying much and cooking the rest to form a metamorphic rock. New volcanoes form on top of this great mountain range.
Erosion reduces the great Andean style mountain ranges.
The giant magma bodies cool into large masses of igneous rock. These large structures are called batholiths and continued erosion eventually brings them to the surface. The volcanoes of previous times are eroded away and are replaced by new volcanoes. The Farallon Plate continues to slide slowly beneath the North American Plate.