IN ROCKS OF DISSIMILAR ORIGIN. Bal 
vitreous rocks, such as pitchstones and obsidians, spherules are often 
developed in great profusion. These appear to represent devitrifi- 
cation around certain points; and similar bodies may sometimes be 
observed in window-glass. Spherules in vitreous rocks usually have 
a radiately fibrous or incipient crystalline structure, often accom- 
panied by a concentric zonal banding which is generally peripheral, 
or is, at all events, more strongly perceptible towards. the margin. 
What starts the devitrification in these spots, and what constitutes 
their nuclei, are questions which yet remain to be worked out; but 
in some instances minute crystals or grains of magnetite &c. may 
be seen to occupy the centres of these bodies. They are not related 
to amygdaloidal, concretionary, or perlitic structures, and, if they 
have any representatives, they are probably crystallites, or rudi- 
mentary phases of aborted crystals, such as those already alluded to. 
In some rocks they are seen to run in regular planes or bands, and 
occasionally coalesce and form continuous belts. 
Then, again, there is perlitie structure, upon which Prof. Bonney 
and Mr. Allport have already written able papers, demonstrating 
its relation to spheroidal structure. Then there is the coarse sphe- 
roidal structure which is at times associated with columnar structure, 
while occasionally it seems to occur in an independant manner; and, 
in the latter instances, it might not be an altogether unprofitable 
inquiry to attempt to ascertain the relation which ordinary con- 
choidal fractures bear to these spheroidal surfaces. The microscopic 
cavities filled with gas or vapour, which are frequently spherical, 
and the little cavities containing fluids and sometimes vitreous 
matter, and which are seldom spherical, do not materially influence 
the broader questions which I now wish to raise, although, when 
present, they should never be ignored. 
Finally we have concretionary structure, commonly giving rise to 
spherical, spheroidal, or lenticular forms of most variable dimensions, 
ranging from minute oolitic bodies up to the large balls which are 
met with in slate-quarries and the ironstone nodules which occur in 
the Coal-measures. 
Ihave by no means enumerated all the varieties of structure 
which come under these different heads, since in this paper I am 
anxious to deal with minutiz only when they affect principles. 
The following table is an attempt to-classify the component parts 
of rocks :— 
Heanite megascopic. 
( microscopic. 
Crystallizations ... ‘macrocrystalline )\ 
indefinite... 4 microcrystalline | 
| eryptocrystalline t 
PVINGROLCIGIMCMNAULOM: ss eececceccscsesseuceerneccccseseacbsongs 
AOE PROMS TAAGLEN!), .)deiusucsecsvuscaesstvevesseedencoescts 
(interstitial (cements in clastic rocks). 
| venous. 
Infiltrations ...... 4 amygdaloidal. 
| internal casts of fossils, 
| metasomatic, 
Ground-masses and in- 
terstitial residues. 
