THE WARWICKSHIRE COAL-FIELD. 6389 
ground-mass has been much altered, and now consists of a fine 
granular substance, partly serpentinous in character, with here and 
there a little calcite. 
The second example is quite similar in texture to the first; the 
felspar, still easily recognizable as triclinic, is far from clear, having 
been partially converted into a grey pulverulent substance. The 
hornblende occurs in various stages of alteration ; some crystals are 
but slightly attacked, while others are to a considerable extent con- 
verted into a pale green serpentinous substance. The alteration has 
followed the cleavage-lines and fractures, and has also invaded the 
substance of the crystals on each side; while the numerous cavities 
just mentioned are also filled by the same substance. In a single 
slice there may be seen almost every degree of change from a slight 
marginal erosion to a mere skeleton of the original. Of the latter, 
however, some little is always left ; and whether the alteration be 
little or great the original crystalline forms are perfectly preserved. 
In the third specimen the alteration has proceeded still further, the 
whole of the hornblende crystals having been completely converted 
into pale green pseudomorphs; they were originally rather large 
and well developed, and their forms are still perfectly sharp and 
distinct. The felspar is here quite turbid and opaque, and the in- 
terstitial ground-mass is represented by calcite. 
It may here be well to observe that the importance of a series 
of specimens like those just described can hardly be overestimated ; 
in fact a collection of specimens in various stages of alteration is 
absolutely essential for any one who wishes to acquire an accurate 
knowledge of the older rocks; and it fortunately happens that a 
diligent search will very frequently supply the requisite materials 
for study. Among the older rocks, which have been quarried to 
some extent, it is generally possible to obtain specimens in every 
stage of alteration ; and a careful study of such examples frequently 
renders it easy to determine the former presence (in other rocks) of 
minerals whose original composition and appearance may have been 
entirely changed. In other words, it will be found that many pseu- 
domorphs possess characteristic microscopic features which render 
their recognition easy to an experienced and cautious observer. I 
venture to urge this point, as there is no more promising field for 
microscopic research ; it is one which has, however, been treated 
with comparative neglect, and even the very existence of extensive 
pseudomorphic changes has not long since been denied by at least 
one writer of eminence. 
DioRITE CONTAINING AUGITE AND OLIVINE. 
Purley Park, near Atherstone.—A portion of the mass in this 
locality is in an excellent state of preservation ; it is a greyish-black 
rock, distinctly crystalline in texture, and in external appearance 
closely resembles a fine-grained dolerite. 
A thin slice exhibits under the microscope a mass of plagioclase 
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