THE MICROSCOPE IN GEOLOGY. 
367 
DESCRIPTION OF PLATES. 
PLATE XVII. 
Fig. 1. Section of greyish black slaggy lava from Etna, magnified 25 
linear. Although this rock appears perfectly amorphous to the eye, it is here 
seen to he composed of crystals of greenish brown augite, colourless felspar, 
and opaque magnetic oxide of iron ; the interstices being filled up with an 
admixture of the same minerals less distinctly crystallised. This specimen 
was, by means of a pole, lifted by the author, on the 21st of May, 1865, out 
of the grand current of lava then flowing out from the north side of the 
crater of the volcano. 
Fig. 2. Section of lava from Vesuvius, magnified 12 linear. This speci- 
men was taken by the author from the current of a.d. 79 covering Hercula- 
neum, and was a somewhat porous, but hard dark grey rock, containing 
abundant crystals of augite, which are well shown in section. 
Fig. 3. Section of a volcanic rock, magnified 12 linear, consisting of 
felspar with some olivine and magnetic oxide of iron, with numerous crystals 
of a pyroxenic mineral, all well shown in this section. The specimen was 
taken by the author from the Valley ^of Papenoo, in Tahiti. 
Fig. 4. Section of the pitchstone occurring in dykes breaking through 
the new red sandstone in the island of Arran, magnified 75 linear. In 
external appearance it resembles a dirty green bottleglass, but under the 
microscope shows a beautiful arborescent crystallisation of a green pyroxenic 
mineral, embedded in the colourless and transparent felspar base. 
Fig. 5. Section of a portion of a basaltic column from Poukhill Quarry, 
near Walsall, Staffordshire, magnified 30 linear. This rock intrudes into 
and disturbs the coal measures, and is seen to be composed of crystals of 
felspar, augite, and titanoferrite, with a little of a green mineral, probably 
the variety of augite called seladonite. 
Fig. 6. Section of diorite (auriferous), taken by the author from near 
Tres Puntes, in the Desert of Atacama, Chili, magnified 30 linear. It con- 
sists of felspar with hornblende, and, in the section here represented, groups 
and isolated crystals of iron pyrites are seen, which frequently accompany 
this rock, and with which the gold is associated. 
Fig. 7. Section of uralite porphyry, from Predazzo, Tyro], magnified 12 
linear. It is composed of felspar with uralite, a mineral having the ex- 
ternal crystalline form of augite, with the chemical composition of horn- 
blende. 
Fig. 8. Section of hornblende schist, cut at right angles to the plane of 
foliation, and magnified 12 linear. It is composed of hornblende and quartz, 
the latter recrystallised, but still showing traces of the outlines of the original 
sand grains : upon examination under a higher power, the quartz will be 
seen to contain numerous fluid cavities. The parallelism of the foliation, 
although perfect when viewed on the large scale, is naturally somewhat 
indistinct in a fragment so small as can be taken into the field of the 
microscope. It is evident, upon inspection, that the structure of such a 
rock is totally different from that seen in the sections of eruptive rocks. 
