If it were a verb, "to twomble" would be to make small marks, scribbles, scratches, dabs and squiggles of crayon, pencil and paint on a canvas. For more than half a decade, Twombly has been working in a vocabulary of elliptical signs - slanting hearts, sexual organs, blobby orbs and splodges of squishy paint applied with brush and finger, and loops of "automatic" writing. His art is primitive, rough, messy; he is the cave painter of abstract expressionism. Yet his pictures are also - for those of us who fully understand the art of twombling - graceful, magisterial and delicate.