Petrology of continental rocks

The aim of petrology, as I see it, is to study the »beginning», development, alteration, intergradation, and the reconstruction of rocks.Geologists know that rocks are not so petrified and still as most people think, but full of movements and action.Much like a biological species, so a rock species has a »life» and a history from cradle to grave.The continental rocks are, of course, the rocks that build up the continents -they occupy about one third of the surface of the earth and are different from the rocks of the ocean floors.They form a crust or just a skin, 30-40 km thick, floating on top of the sima of basic or ultrabasic composition.Under the ocean the sima ascends to within a few kilometers of the surface.Thus the continental rocks lie in extensive, shallow depressions in the simatic shell of the globe.A glance at figure 1 shows that the continental rocks represent a very minor part indeed of the globe.