These form the most important part of the subject from the geological point of view, because they contribute largely to the permanent materials of the earth's crust. We meet with such materials of all geological ages, sometimes developed on a vast scale. The study of volcanic products is the key which enables us to comprehend the great group of rocks which are called igneous, though, as we shall see later, by no means all of these were poured out on the surface of the ground.
Fig. 19. - Edge of Hale-mau-mau, showing the ropy forms of the highly fluid lava, when cooling. (Photograph by Libbey).
Volcanic products are of three kinds: (1) lava, or molten rock; (2) fragmental material, including blocks, lapilli, bombs, the so-called volcanic ashes, cinders, and the like; (3) gases and .vapours.
Fig. 20. - Ropy lava, Vesuvius.