ROCKS 53 
enstatite, or biotite, we have augite-, hornblende-, enstatite-, or mica- 
picrite; the picrites are sparingly represented in the British Islands ; 
Lherzolite (from L’herz in the Pyrenees)=olivine+enstatite+light green 
pyroxene, with some accessory spinelloid and a little magnetite and apatite. 
The magma-basalts are volcanic rocks, mostly of late 
Tertiary age, occurring, like ordinary basalt, both effusively 
and intrusively. The peridotites are usually intrusive, and 
closely related to the dolerites and gabbros, into which they 
graduate by the increase of felspathic constituents. Bosses of 
gabbro and dolerite are not infrequently bordered by olivine- 
rock, into which they pass—the two kinds of rock obviously 
being different phases of one and the same intrusive mass. 
The olivine-rocks are often highly serpentinised—the 
olivine being more or less readily altered. Thus many 
massive serpentines are merely highly altered igneous rocks 
(See p. 23). 
B. Fragmental Igneous (Pyroclastic) Rocks 
These rocks include the various kinds of material which 
have been ejected from volcanoes in the form of blocks, 
scoriz, lapilli, bombs, sand, and ash. Blocks and Lapilli are 
the names given to the larger and smaller rock-fragments, 
which may be angular or subangular; while the finer grained 
materials are known as Sand and Ash. Scoriz are loose 
pieces of cindery lava (Plate XVI.). Bombs are elliptical 
or pear-shaped fragments, often vesicular or hollow (see 
Eiaces XVII, XVIII.). They are simply. clots torn from 
the surface of a mass of molten rock by the explosive energy 
of steam, and ejected from the crater of a volcano, their 
form being doubtless the result of their rotatory motion. 
Accumulations of these and other kinds of volcanic ejecta 
frequently become indurated, and are met with in regular and 
irregular beds associated with crystalline igneous rocks of all 
geological periods. Volcanic Agglomerate is the name given 
to a coarse admixture of large and small blocks and stones 
set in a matrix of comminuted rock débris and grit, which 
may be either abundant or meagre. Frequently, this rock is 
found occupying the pipes or throats of ancient volcanoes— 
the upper portions of which have been denuded away. 
