There is another way in which small particles of lava are formed. Sometimes large pieces of hardened lava are shot upwards into the air with a velocity as great as that with which a heavy projectile leaves the muzzle of a large gun. These heavy particles striking against one another, either while rising or falling, are broken into smaller fragments. Sometimes, indeed, these fragments fall back again into the crater from which they are again violently thrown out, and are again broken into smaller fragments either while rising or falling. You will, probably, remember several instances of volcanic eruptions where masses of rock were thrown violently up into the air out of the crater. These larger masses are known as _volcanic blocks_. They probably consist of masses of hardened lava that have collected in the tube of the volcano during some of its periods of inactivity. Sometimes, however, they consist of fragments of rocks that are not of volcanic origin. Cases are on record where volcanic blocks have been thrown out of the craters in so great quantities as to cover the surface of many square miles of land with fragments hundreds of feet deep. There is sometimes formed on the surface of a pool of lava as it collects in the craters of such volcanoes as Mt. Loa or Kilauea, when the volcanoes are not in eruption, a material resembling froth or scum. The same thing sometimes occurs on the surface of some kinds of lava as it runs down the side of the mountain. In this way a very light variety of highly cellular lava known as _pumice stone_ is produced. The action which thus takes place is not unlike that which occurs during the raising of a lot of the dough from which bread is made, where the carbonic acid gas which is formed during the raising of the dough expands, and produces the well-known open cellular structure of well-raised bread. In the case of pumice stone, however, this raising goes on to such an extent that the mass consists often of less than 2% of solid matter, the remainder being a tangled mass of air. [Illustration: THE LAVA FLOW OF THE CRATER OF KILAUEA, HAWAIIAN ISLANDS _From a Stereograph, Copyright, by Underwood & Underwood_] Fragments of lava that possess a cellular structure form what are known as _scoriæ_. The lightest of all kinds of scoriæ is what is known as _thread-lace scoriæ_. Here the thin walls consist of mere threads. Figs. 26 and 27 represent the appearance of thread-lace scoriæ from Kilauea. The separate threads are very fine, being only from one-thirtieth to one-fortieth of an inch in thickness. As can be seen, this form of scoriæ have six-sided or hexagonal shapes. You can form some idea of the great lightness of such scoriæ when you learn that they contain only 1.7% of rocky material. Indeed, they contain so little solid material that a layer of volcanic glass only one inch thick, if blown out into scoriæ, would be able to produce a layer sixty inches thick.