COMMUNITY OF STRUCTURE IN ROCKS OF DISSIMILAR ORIGIN. 327 
27. On Community of Srrucrure in Rocks of DissrmmaR ORieqn. 
By Frank Rurrny, Esq., F.G.8., H.M. Geological Survey. 
(Read January 22, 1879.) 
SPECULATIONS upon the origin of rocks have always been fraught 
with more or less grave difficulties; and although much has from 
time to time been written upon the subject, those writings are in 
very many instances open to the objection that they are merely 
speculations, and that the data from which they have been educed 
are often by no means of a comprehensive nature. Within the last 
few years the microscope has afforded a large amount of information 
respecting peculiarities which were previously unknown in the struc- 
ture of rocks—#mucroscopic peculiarities it is true, but still amply 
sufficient to demonstrate that the cubic inch and the cubic mile of 
rock are often identical in mineral constitution, and built up in the 
same manner. 
Given a patent community of origin, and often a coexisting com- 
munity in the mineral constitution of rocks, we then see little or no 
reason to doubt the truth of many of the deductions which have 
been made; and the geological literature of twenty years ago then 
appears to be often quite as satisfactory as that of to-day. 
It is, however, very common to find that the origin of certain 
rocks is by no means so evident, and the darknéss which veils their 
past history, and the conditions under which they were formed, is 
not always to be dissipated even by the most patient and unbiassed 
research. 
So far as the nomenclature of rocks is concerned, it was a wise 
practice on the part of the older geologists to avoid entering too 
much into detail in the separation of rocks which were more or less 
closely allied in mineral constitution ; and, instead of blaming, we 
should rather thank them for the employment of such generally 
comprehensive terms as greenstone, aphanite, &c., which roughly 
indicate the facies of certain rocks without implying toa definitely 
a knowledge reserved for the students of a later date. 
That this knowledge is still very imperfect there is no doubt ; and 
it behoves modern petrologists to exercise a like cautiousness, and 
to beware lest, in discarding some of the useful terms employed by 
their predecessors, they institute a more complex classification, 
hemmed in by definitions sharper than any which nature warrants— 
definitions which a future generation may bring in evidence against 
them. 
Sharp definitions are, however, to a certain extent a necessary 
evil. Without them no satisfactory classification can exist, and 
without classification no student can work systematically. 
It may be well to consider a few petrological terms and definitions, 
and to endeavour to ascertain how far they tend to elucidate the 
structure, the mineral constitution, and the origin of certain rocks, 
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