
Parr. Sror.i.§1] VOLCANIC PRODUCTS. 205 
light soils; but the obsidians present rugged black sheets of rock 
roughened with ridges and heaps of grey froth-like pumice. Some of 
the most brilliant surfaces of colour in any rock scenery on the globe 
are to be found among volcanic rocks. The walls of active craters 
elow with endless hues of red and yellow. The Grand Cajon of the 
Yellowstone River has been dug out of the most marvellously tinted 
lavas and tuffs. 
4, Fragmentary Materials.—Under this title we include all 
- the substances which, driven up into the air by volcanic explosions, 
fall in solid form to the ground—the dust, ashes, sand, cinders, and 
blocks of every kind which are projected from a volcanic orifice. 
These materials differ in composition, texture, and appearance, even 
during a single eruption, and still more in successive explosions of 
the same volcano. For the sake of convenience separate names are 
applied to some of the more distinct varieties, of which the following 
may be enumerated. 
(1) Ashes and Sand.—In many eruptions vast quantities of 
an exceedingly fine light grey powder are ejected. As this substance 
greatly resembles what is left after a piece of wood or coal is burut 
in an open fire, it has been popularly termed ash, and this name has 
been adopted by geologists. If, however, by the word ash the result 
of combustion is implied, its employment to denote any product of 
voleanic action must be regretted as apt to convey a wrong impression. 
The fine ash-like dust ejected by a volcano is merely lava in an 
extremely fine state of comminution. So minute are the particles 
that they find their way readily through the finest chinks of a closed 
room, and settle down upon floor and furniture as ordinary dust does 
when a house isshut up. From this finest form of material gradations 
may be traced, through what is termed volcanie sand, into the coarser 
varieties of ejected matter. In composition the ash and sand vary 
necessarily with the nature of the lava from which they are derived. 
Their microscopic structure, and especially their abundant microliths, 
erystals, and volcanic glass have been already referred to (p. 162). 
(2) Lapilli or rapilli are ejected fragments ranging from the 
size of a pea to that of a walnut, round, subangular, or angular in 
shape, and having the same indefinite range of composition as the 
finer dust. As a rule, the coarse fragments fall nearest the focus of 
eruption. Sometimes they are solid fragments of lava, but more 
usually they have a cellular texture, while sometimes they are so 
light and porous as to float readily on water, and when ejected near 
the sea, to cover its surface. Well-formed crystals occur in the lapilli 
of many volcanoes, and are also ejected separately. It has been 
observed indeed that the fragmentary materials not infrequently 
contain finer crystals than the accompanying lava.’ 
(3) Volcanic Blocks are larger pieces of stone, often angular 
in shape. In some cases they appear to be fragments loosened from 
already solidified: rocks in the chimney of the volcano. Hence we 
1 §. von Waltershausen, Island und Sicilien, 1853, p. 328. 
