SUMMARYThe study of the hydraulic behaviour of mica and particularly the study of the modalities of the fall of particles in a motionless liquid presents a great variety of types and a vast series of falling speeds. The observations made by McNown(1953), concerning grains, show that there are various modalities.The falling speed differs greatly from that indicated by the usual sedimentation speed curves, because the calculation of the nominal diameter in which the grain is compared to a sphere differs considerably from reality.The calculation of the drag coefficient (Cn) with regard to the Reynolds number (Re) again accentuates this disparity.