Se ere of Rocks and Minerals. 99 
made till their mineral constituents are accurately ascertained ; and 
in the case of fine-grained or compact rocks, this cannot be done, 
either by chemical analysis, or other ordinary methods of observa- 
tion. In such cases, various expedients have been adopted, but only 
approximative results were obtained, and that they were altogether 
unsatisfactory will be evident from the following remarks of Bis- 
chof.* “It appears,” he says, “to be quite in vain to attempt a 
classification of Greenstones ; they constitute a series, the extreme 
members of which are, on the one hand, Augitic Porphyry, or Mela- 
phyr, with distinguishable Augite and Labradorite; and on the 
other Diorite or Syenite, with distinguishable Hornblende. Between 
these opposite extremes there are many intermediate varieties, with 
regard to which it is quite uncertain whether they are to be classed 
with augitic or hornblendic rocks.” Again, Cotta T describes Apha- 
nite, as “Trap in part, Melaphyr in part, a compact apparently 
homogeneous mass. The separate ingredients of this rock are not 
to be recognized by the naked eye, hence the name Aphanite given 
by Hatiy. The minuteness and intimate union of the ingredients 
of Aphanite when quite compact, make it impossible with the 
ordinary aids to discover whether it belongs to Diabase, Gabbro, or 
Diorite.” Now it appears to me that to call a rock Aphanite, or to 
say that a classification of Greenstones is impossible, is Just as absurd 
as it would be for a zoologist or botanist to assert that no satis- 
factory account could be given of animal or vegetable tissues because 
they cannot be made out by the naked eye or a pocket lens. The 
examination of many rocks, if not too fine grained, may be facilitated 
by examining a polished surface with a low power, or even by 
sunply wetting the freshly-broken surface instead of polishing ; but 
in the case of basaltic and other fine-grained rocks it is necessary 
to prepare thin sections for examination by transmitted light. This 
method is in fact by far the most satisfactory in the majority of 
cases ; for not only may the various constituent minerals be readily 
observed, but the order in which they crystallized out from the 
mass, the changes which they have frequently undergone during 
the long lapse of ages, and other important facts, may also be thus 
ascertained. 
A microscopical examination of rocks shows that, as a rule, the 
igneous rocks may be distinguished at once from all others by their 
structure, which is that of a more or less perfect network of minute 
crystals: in many cases all the minerals are well crystallized; in 
others there is an amorphous or glassy base in which they are 
enclosed ; there are, however, other rocks, such as the Felstones and 
the more recent volcanic Phonolites some of which do not present 
this crystallized arrangement of their constituents; and then there 
are the Porphyrites, which are characterized by the presence of 
* ‘Chemical Geology,’ vol. iii., p. 300. t ‘Rocks Classified,’ p. 157. 
