FRAGMENTAL PRODUCTS 5 I 



thousands of miles. The very large blocks are commonly frag- 

 ments 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 measur- 

 ing nine feet in diameter. 



More important and much more extensively formed and widely 

 spread are those fragmental products which are derived from the 

 lava itself. 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 spheri- 

 cal form and are called volcanic bombs. Lapilli are smaller, 

 rounded fragments, and volcanic ashes and dust are very fine par- 

 ticles, though with a wide range of variation in size. The term 

 ashes is so far unfortunate, that it implies combustion, but never- 

 theless 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 aggloj?ierale, 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 Na- 

 tional Park, and in many other parts of the Rocky Mountain 

 region. The finer accumulations of ash, formed at a greater dis- 

 tance from the vent, are roughly sorted by the air and often quite 

 distinctly divided into layers. The torrents of rain, which so fre- 

 quently accompany volcanic outbursts, gather up the finer parti- 

 cles, forming sheets and streams of hot mud, which on drying 

 sets into quite a firm rock, called tuff or tufa. Herculaneum lies 

 buried under a solid mass of such tuff more than sixty feet deep, 

 and hence is much less accessible than Pompeii, which is covered 

 with scoriae, lapilli, and ashes. 



