308 #. RUTLEY ON COMMUNITY OF STRUCTURE 
actually been seen to fall. We may admit that in many cases such 
assertions are backed by great probability and are very likely correct ; 
but, in the present state of knowledge, we should often accept them 
rather as theories than as facts. 
The writings of Zirkel, Vogelsang, Von Lasaulx, Rosenbusch, and 
Penck have to some extent helped us to a right understanding of 
the relative points of resemblance and difference between volcanic 
ejectamenta and other rocks which may assimilate to them either 
in mineral constitution or in structure ; but apparently there is much 
more to be learned before we can safely speculate on the pyroclastic 
origin of many of our older rocks. 
Having now considered some of the elements of doubt which 
frequently perplex us, let us look for a few of the phenomena which 
may serve to dispel this uncertainty, first with regard to clastic and 
pyroclastic rocks, and next in the case of grits, felstones, and once- 
vitreous lavas. shied 
(i) A pyroclastic rock may sometimes be distinguished from an 
ordinary clastic rock by the presence of crystals which are more 
especially characteristic of lavas and which, as a rule, would become 
decomposed prior to the atmospheric disintegration of the lavas which 
contain them. ‘This, however, is not always a trustworthy character, 
since it is often difficult to distinguish between tuffs and consolidated 
ashes. A safer means of discriminating between pyroclastic rocks 
and ordinary clastic rocks lies in the occurrence in the former of 
crystals with fused surfaces or with vitreous envelopes, and of isolated 
shreds of vitreous matter. These are the best evidence ; while in 
the next place a paucity or absence of quartz may also be regarded as 
in favour of pyroclastic origin so long as the quartz does not appear 
to have been derived from its dissociation from silicates, in which 
latter case felsitic matter would probably be the result. 
In many cases it appears quite impossible to distinguish a consoli- 
dated ash from a volcanic tuff. Indeed atmospheric degradation and 
voleanic eruption going on synchronously, it is quite possible to 
find rocks of a mixed character—rocks, in fact, which are partly 
tufaceous and partly ashy; and these, again, are sometimes mixed 
with ordinary sediments. 
(ii) A felspathic grit may be distinguished from a felstone dyke 
by its mode of occurrence, and from an interbedded felstone by the 
occasional fragmentary character of the felspar crystals &e. and 
by the presence of a cement, which often is of a friable nature, 
rendering the fracture or disintegration of the rock an easy matter. 
(iii) The characters which may serve to distinguish a deyitrified 
or felsitized pitchstone, obsidian, perlite, or rhyolite from a felstone 
are :—Fluxion structure, especially when indicated by bands or the 
remains of bands of unaltered glass, or when indicated by devitrified 
bands which differ from the surrounding matter in microcrystalline 
or cryptocrystalline development or texture, or, again, as indicated 
by streams of microlites following definite directions. 
The presence of perlitic structure. 
The presence of spherules lying in bands. 
