360 
POPULAR SCIENCE REVIEW. 
only allows of a mere mention of its application. In distin- 
guishing dolerites from diorites, when fine-grained, as is often 
of considerable geological importance, the fibrous structure of 
the hornblende of the latter is generally so well developed (PI. 
XVII. fig. 6), even when present in very minute quantity, as to 
distinguish it readily from the augite of the former, which pos- 
sesses no such structure. Even in the case of Uralite, a mineral 
characteristic of certain porphyritic rocks, which has the ex- 
ternal form of augite, although its chemical composition is that 
of hornblende, the fibrous structure characteristic of hornblende 
is distinctly visible (PI. XVII. fig. 7). The microscopic struc- 
ture of some minerals, however, varies with their origin ; thus 
Sorby has shown that the structure of augite, and some other 
minerals in meteorites, is quite distinct from that of the same 
minerals occurring in eruptive rocks, and demonstrated, in a 
very striking manner, how the study of such peculiarities is 
likely to clear up the mystery in which the origin of these bodies 
is involved. 
When, as is often the case, especially with translucent, colour- 
less minerals like quartz, leucite, calcite, felspar, &c., the appear- 
ance presented under the microscope is alike, their optical 
properties and the use of polarised light afford the means of 
distinguishing between them with certainty; as, also, in the 
event of one substance being present under two forms, as calcite 
from aragonite, monoclinic from triclinic felspars, &c. In a 
similar manner, the structure, whether crystalline or vitreous, is 
determined, and valuable information obtained, elucidating the 
mode of formation and origin of the rocks themselves. 
The alterations produced in eruptive rocks subsequent to 
their solidification, by the action of water, atmospheric or other 
agencies, are studied with advantage under the microscope. For 
want of space, a single example can only be here given, and is 
depicted on PI. XVIII. fig. 12. 
In this section, the skeleton of labradorite is seen remaining 
as evidence of the original crystallised structure, whilst the 
interstices contain the products of the alteration of the more 
easily decomposable augite, the structure of which is nearly ob- 
literated, and part of its lime converted into carbonate. The 
rock in question is the so-called “ white horse ” of Staffordshire, 
found embedded in or breaking through the coal measures, 
which are frequently burnt or altered at points of contact with 
this rock, which itself often has the appearance of a whitish clay. 
The origin of this rock, whether sedimentary or igneous, was 
disputed until the more recent geological and chemical examina- 
tions of it have proved satisfactorily its identity with the Eowley 
basaltic rock, very similar to that of Poukhill (PI. XVII. fig. 5). 
PI. XVIII. fig. 13, is a section of a crystalline slag produced in 
