Classification of rocks

You will probably only see one type of rock, at least in the first place . You can distinguish small mineral grains from each other, the most common being milk white (feldspar) and more shiny, transparent white (quartz). The black spots are probably from a mineral called biotite, but can also be amphibole. All these minerals are very common. The rock is massive with no specific structures. The photo below shows a small section of rock in one of the boulders on the beach. Compared with the description of the different types of rocks, one can conclude that this probably is a granite, which also agrees with more thorough studies.

You do, however, also see small inclusions of another rock in the otherwise massive main rock. These have normally layered and often folded structures. If you read the description of rocks, you might have found out that the inclusions are of gneiss.

Such a mixture of rocks is uncommon. The granite has probably been formed when gneiss melted to a lava deep under the earth’s crust and became mixed with pieces of soft, nearly melted gneiss , in the Caledonian orogeny c. 420 million years ago.