This division includes all the materials which are ejected from the volcano in a solid state. These are of all sizes and shapes, from huge blocks weighing many tons, down to the most impalpable dust, which the wind will carry for thousands of miles. The very large blocks are commonly fragments of the older rocks through which the volcanic vent has burst its way, tearing a great hole and scattering the fragments widely. For fifteen miles around the lofty volcano of Cotopaxi in Ecuador lie great blocks of this nature, some of them measuring nine feet in diameter.

Volcanic bomb, showing scoriaceous texture; about 2/3 natural size.

Fig. 27. - Volcanic bomb, showing scoriaceous texture; about 2/3 natural size.

More important and much more extensively formed and widely spread are those fragmental products which are derived from the lava itself and are due to the sudden and explosive expansion of the vapours and gases with which the molten mass is intimately commingled and saturated. The more violently explosive the eruption, the greater the proportion of the lava that will be blown into fragments. In such eruptions as that of Krakatoa, all of it is thus dispersed and none remains to form lava flows- Cindery fragments thrown out of the vent are called sconce, while portions of still liquid lava thus ejected will, on account of their rapid rotation, take on a spheroidal form and are called volcanic bombs. Lapilli are smaller, rounded fragments, and volcanic ash and dust are very fine particles, though with a wide range of variation in size. The term ash is so far unfortunate that it implies combustion, but nevertheless it accurately describes the appearance of these masses.

In the immediate neighbourhood of the vent fragments of all sizes accumulate, but the farther we get from the volcano, the smaller do the fragments become. The coarser masses around the vent form a volcanic agglomerate, in which the fragments are of all shapes and sizes, heaped together without any arrangement. More regular sheets of large angular fragments form volcanic breccia, and these may be seen on a grand scale in the Yellowstone National Park, and in many other parts of the Rocky Mountain region. The finer accumulations of ash, formed at a greater distance from the vent, are roughly sorted by the air and often quite distinctly divided into layers, while, as already explained, the muds on drying set into quite a firm rock, called tuff or tufa.

As volcanoes so generally stand in or near the sea, and as the lighter fragments, such as pumice, often drift for months upon the water before they sink, while the finer dust is carried vast distances by the wind, it would naturally be expected that volcanic materials should have a very wide distribution upon the sea-bottom. Such, indeed, proves to be the case, and this kind of material, laid down in the sea, has formed important rock masses in nearly all the recorded ages of the earth's history. The exact character of the rock formed in this fashion will be governed by various circumstances, such as the fineness and abundance of the material, whether it is showered into quiet waters or along a wave-beaten coast, whether and in what proportion it is mingled with sand or mud. When the volcanic ash preponderates, a tuff is formed, very much like those which accumulate on land, but more regularly stratified.

The fragmental volcanic products, whether coarse or fine, retain their characteristic texture and appearance, so as to be readily recognizable, though perhaps only with the microscope. The great bulk of these materials consists of lava shattered by the steam explosions and quickly chilled. The coarser fragments display the frothy and vesicular nature of scoriae, while the finer particles are glassy or crystalline. Mere comminution of the mass does not change its essential texture.

It will be readily imagined that lavas very rarely contain fossils. Though the flows often overwhelm living beings, the intense heat at once destroys them, seldom leaving a trace behind, though charred tree-trunks are sometimes recognizably preserved. In tuffs, on the other hand, fossils, especially those of plants, are frequently well preserved, and tuffs formed under water have fossils as abundantly as any other aqueous rocks.