They are probably gneiss, a banded metamorphic rock.
Gneiss shows such banding. Alternating bands are known to Geologists as Gneissose Banding, and is used to instantly classify the metamorphic rock.
It could be foliated metamorphic rock or sedimentary rock, depending on its mineral makeup and method of formation.
Foliated metamorphic rocks.
It could be a sedimentary rock or a foliated metamorphic rock exhibiting alternating bands of light and dark minerals.
gneiss