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162 GEOGNOSY. ; -[Boox II. — 
a 
the same way. In practice, however, these detrital reconstructed — 
rocks cannot always be certainly distinguished from those which have — 
been formed by the consolidation of true volcanic dust and sand. Their 
chemical and lithological characters, both macroscopic amd micro- 
scopic, are occasionally so similar, that their respective modes of origin 
have to be decided by other considerations, sueh as the occurrence of 
lapilli, bombs, slags in the truly volcanic series, and of well water- 
worn pebbles of volcanic rocks in the other. Attention to these 
features, however, usually enables the geologist to make the dis- 
tinction, and to perceive that the number of instances where he may 
be in doubt is less than might be supposed. Only a comparatively 
small number of the rocks classed here are not true voleanic ejections. 
Referring to the account of volcanic action in Book III. Part I., 
we may here merely define the use of the names by which the 
different kinds of ejected volcanic materials are known. 
Voleanie Blocks.—Angular, sub-angular, round, or irregularly- 
shaped masses of lava several feet in diameter, sometimes of uniform 
texture throughout, as if they were large fragments dislodged by 
explosion from a previously consolidated rock, sometimes compact in 
the interior and cellular.or slaggy outside. 
Bombs.— Round, elliptical, or discoidal pieces of lava from a few 
inches up to one or more feet in diameter. They are frequently 
cellular internally, while the outer parts are fine grained. Ocecasion- 
ally they consist of a mere shell of lava with a hollow interior like 
a bomb-shell. Their mode of origin is explained at p. 206. | 
Lapilli (rapilli).—Hjected fragments of lava, round, angular, or 
indefinite in-shape, varying in size from a pea to a walnut. Their 
mineralogical composition depends upon that of the lava from which 
they have been thrown up. Usually they are porous or finely 
vesicular in texture. 
Volcanic Sand, Voleanic Ash.—The finer detritus erupted from — 
volcanic orifices, consisting partly of rounded and angular fragments 
up to about the sizeof a pea, derived from the explosion of lava 
within eruptive vents, partly of vast quantities of microliths and 
crystals of some of the minerals of the lava. The finest dust is in a 
state of extremely minute subdivision. When examined under the 
microscope, it is sometimes found to consist not only of minute crystals 
and microliths, but of volcanic glass, which may be observed adhering 
to the microliths or crystals round which it flowed when still part of 
the fluid lava. The presence of minutely cellular fragments is 
characteristic of most volcanic fragmental rocks, and this structure 
may commonly be observed in the microscopic fragments and 
filaments of glass. | | 
When these various materials are allowed to accumulate, they 
become consolidated and receive distinctive names. In cases where 
they fall into the sea or into lakes, they are liable at the outer margin 
of their area to be mingled with, and insensibly to pass into ordinary 
non-yoleanic sediment. Hence we may expect to find transitional 

