LITHOLOGY. 163 
is particularly interesting. A crystal of augite and one of hornblende 
lie united together, with their prisms parallel to one another, and the 
section is so cut as to intersect the prisms in a plane parallel to their 
bases. This section is represented in Fig. 6 on Pl. 9. Though the out- 
lines of both the crystals are very perfect, they are united together by an 
irregular line. The hornblende crystal is entirely fresh, but the augite 
is decomposed and dissolved away, and its place is now filled with 
decomposition products. But among the chlorite, epidote, and calcite 
are some fragments of hornblende which are indeterminately situated, 
and perfectly angular and fresh. They must have dropped into the cav- 
ity made by the decay of the augite crystal, from the irregular edge of 
the hornblende crystal, at a time when the space was partially empty, 
and have been subsequently enclosed in the new products. No other 
space originally filled with augite contains any hornblende, with the ex- 
ception of this one where the crystals lie together, and the irregularity 
of the edge of the hornblende would make it easy to derive such frag- 
ments from it. This section furnishes a very pretty illustration of the 
stability of hornblende as compared with augite, and was one of the cases 
which was in mind in the discussion of the chemical differences between 
these species when associated. 
But in this kind of diorite the hornblende does sometimes entirely 
decompose. We will next consider a case of this kind. Near the Pro- 
file house there is a dyke of black rock, which, on being examined with 
the microscope, proves to be a diorite which originally had well defined 
crystals of hornblende; but now the spaces bounded by the crystalline 
outlines of hornblende are filled in most cases with a heterogeneous 
mixture of biotite, magnetite, and calcite, while occasionally’'a centre of 
dirty-green hornblende is still preserved. At other, times, only magnet- 
ite and calcite are seen; and in the aggregate the cleavage directions of 
the original mineral are indicated by clearer spaces through the turbid 
mass. The rest of the rock is composed of an aggregate of crystals of 
hornblende, biotite, chlorite, plagioclase, calcite, and magnetite. A sec- 
tion through one of the hornblende crystals is shown in Fig. 3 on Pl. ie 
and another in Fig. 3 on Pl. 2. Externally, this rock has lost the porphy- 
ritic appearance that characterizes most of our diorites, and the reason is 
self-evident. 
