The ashes vary in size from large blocks twenty feet or more in diameter to the minutest impalpable dust. The large masses are called 'volcanic bombs'; they have mostly a rounded, elliptical or pear-shaped form owing to rotation in the air before they solidified. Many of them have ribbed or nodular surfaces, and sometimes they have a crust intersected by many cracks like the surface of a loaf of bread. Any ash in which they are very abundant is called an agglomerate.