502 
ELEMENTS OF GEOLOGY. 
magnesia and oxide of iron; whilst Phlogopite contains still 
more of the former substance. In rocks containing quartz, 
muscovite or lepidolite are most common. The mica in re¬ 
cent volcanic rocks, gabbros, and diorites is usually Biotite, 
while that so common in metamorphic limestones is usually, 
if not always, Phlogopite. 
Amphibole and Pyroxene Group.— The minerals included 
in the table under the Amphibole and Pyroxene Group differ 
somewhat in their crystalline form, though they all belong 
to the monoclinic system. Amphibole is a general name for 
all the different varieties of Hornblende, Actinolite, Tremo- 
lite, etc., while Pyroxene includes Augite, Diallage, Malaco- 
lite, Sahlite, etc. The two divisions are so much allied in 
chemical composition and crystallographic characters, and 
blend so completely one into the other in Uralite (see p. 499), 
that it is perhaps best to unite them in one group. 
Theory of Isomorphism. —The history of the changes of 
opinion on this point is curious and instructive. Werner 
first distinguished augite from hornblende; and his proposal 
to separate them obtained afterwards the sanction of Haliy, 
Mohs, and other celebrated mineralogists. It was agreed 
that the form of the crystals of the two species was different, 
and also their structure, as shown by cleavage —that is to 
say, by breaking or cleaving the mineral wuth a chisel, or a 
blow of the hammer, in the direction in which it yields most 
readil}^ It was also found by analysis that augite usually 
contained more lime, less alumina, and no fluoric acid; which 
last, though not always found in hornblende, often enters into 
its composition in minute quantity. In addition to these 
characters, it was remarked as a geological fact, that augite 
and hornblende are very rarely associated together in the 
same rock. It was also remarked that in the crystalline 
slags of furnaces augitic forms were frequent, the horn- 
blendic entirely absent; hence it was conjectured that horn¬ 
blende might be the result of slow, and augite of rapid cool¬ 
ing. This view was confirmed by the fact that Mitscherlich 
and Berthier were able to make augite artificially, but could 
never succeed in forming hornblende. Lastly, Gustavus Rose 
fused a mass of hornblende in a porcelain furnace, and found 
that it did not, on cooling, assume its previous shape, but in¬ 
variably took that of augite. The same mineralogist ob¬ 
served certain crystals called Uralite (see Table, p. 499) in 
rocks from Siberia, which possessed the cleavage and chem¬ 
ical composition of hornblende, while they had the external 
form of augite. 
If, from these data, it is inferred that the same substance 
