Lac Nègre

If you look closely at this grey rock, you will see that it is made of different minerals. Quartz (grey) and feldspar (white) sometimes big with a few sparkling areas here and there on the rock. This is black mica and more specifically biotite. You have discovered blocks of granite. All the minerals it is made up of are contiguous and visible with the naked eye. The texture is igneous, characteristic of plutonic rocks which crystallised deep in the earth.  

Migmatite is a metamorphic rock which has undergone partial fusion. It is grey and also contains quartz and feldspar, but displays clearly clashing levels. The evidence for this partial fusion is displayed by the rock itself. The small lighter bands whose composition is close to granite, rich in quartz and feldspar: leucosome.

When you reach Lac Nègre, you will see on its right bank, several metres below the lake’s spillway, a granite outcrop which is notable for the presence of several vertical lines which split the granite into fairly regular lozenge shapes.This corresponds to a transformation of the granite. There is a very particular structure with the apparition of cleavage and a big reduction in the size of the minerals. This phenomenon, characteristic of deformed rocks, is known as mylonite.