368 
POPULAR SCIENCE REVIEW. 
PLATE XVIII. 
Em 9. Section of granite from St. Just, Cornwall, magnified 25 linear. 
It consists of crystals of ortlioclase, hexagonal ditto of brown mica, and of 
colourless quartz ; the latter, when examined by a higher power, is seen to 
contain fluid cavities. 
Fm 10. Section of volcanic quartz trachyte from Jacna, Peru, magnified 
10 linear. It consists of quartz crystals, often large and well defined, along 
with smaller hexagonal plates of black mica, scattered through a felspathic 
mass indistinctly crystallised. The quartz of this trachyte also contains 
fluid cavities (seen by employing a higher power), similar to those occurring 
in the quartz of granite. This rock is developed very extensively along the 
volcanic range of the Andes of South America. 
Fig, 11. Section of a volcanic rock, taken by the author from the foot of 
the volcano of Ariquipa, in Peru, magnified 6 linear. This rock is of a grey 
colour, and has a porphyritic structure, arising from crystals of white felspar, 
scattered through a grey base, which the microscope shows to be composed 
of felspar, dark brown-black crystals of augite, hexagonal crystals of almost 
black mica, and a little magnetic oxide of iron. 
Fig. 12. Section of the so-called a white horse ” dykes, intersecting and 
altering the coal measures of Staffordshire, magnified 35 linear. The frame- 
work of felspar crystals still remains unaltered, whilst the other mineral 
constituents are decomposed so far as to be frequently unrecognisable. Its 
chemical composition, in conjunction with the microscopic structure, show 
this rock to be similar to that from Poukhill (PL XVII. fig. 5), more or less 
altered by the action of water. 
Fig. 13. Section of a crystalline slag from smelting silver ores, magnified 
30 linear. The structure is seen to be very similar to that found in many of 
the more felspathic doleritic rocks. 
Fig. 14. Section of fine-grained Silurian (uncleaved) slate, from Sorata, 
in Bolivia, magnified 400 linear. It is seen to be a mere mechanical aggre- 
gate of minute, irregularly weathered, or rounded particles of sand, clay, 
mica, oxide of iron, &c., without any trace of arrangement or crystalline 
structure being visible. 
Fig. 15. Section of Lower Silurian roofing-slate, from Festiniog, North 
Wales, cut at right angles to the cleavage, and magnified 200 linear. This 
shows that the parallel structure of cleavage is due entirely to the mechanical 
arrangement of the unaltered particles of the rock, and not to the develop- 
ment of any crystalline structure. 
Fig. 16. Section of a highly indurated bed in the upper oolitic series at 
Iluaylillos, Peru, magnified 30 linear, and in external appearance so much 
resembling an eruptive rock as to have been mapped as such by D’Orbigny. 
The microscopical examination, however, at once shows its true sedimentary 
character, and resolves it into a mere mechanical aggregate of quartz, "sand, 
&c., without any of the crystalline character peculiar to eruptive rocks. 
