This paper deals with the Arroyo Farandón (Asturias) igneous sills, which intruded within the sedimentary rocks of Oville Formation. They have alkaline composition and are classified as trachytes. As a consequence of a new detailed survey, continuous zonation has been distinguished along the main sill, which show outstanding developing of vesicular-amigdalar zones at the top and bottom of the body, suggesting vesiculation processes controlled by temperature gradients. Slight geochemical variations across the sills have been recorded