6 
POPULAR SCIENCE REVIEW. 
cient to recognise them. It can, however, be shown that the 
microscopical structure of most minerals will afford characteristic 
marks by which they may be readily known. Even in the case 
of transparent, colourless minerals like rock crystal (quartz), 
sanidine, leucite, &c., when the microscopical appearance is 
nearly identical, “ their optical properties, and the use of pola- 
rised 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.” (Forbes). Many rocks have such a 
minute, close-grained structure, that without the microscope it 
is impossible to learn anything of their composition; but as 
soon as we have learnt the characteristic appearance of various 
minerals in their microscopical forms, we have at once the 
power to unravel the details of the structure, and the history of 
the finest-grained rocks, be they slates or basalts, or any analo- 
gous forms. The microscope, for instance, it has been well 
pointed out, will enable us easily to distinguish between a 
diorite and a dolerite : the hornblende of the former, as soon as 
we know the microscopical behaviour of that mineral, cannot 
possibly be mistaken for the augite, which replaces the horn- 
blende in a dolerite. 
Again, the microscope shows us how closely allied are the 
older volcanic rocks, the so-called trappean rocks of palaeozoic 
date, the tertiary anamesites and basalts, and the products of 
still active volcanoes. Certain common structures are seen to 
prevail throughout them all. We may, for instance, find in 
one specimen of dolerite “ a close network of crystals in actual 
contact without any intervening cement ; in another we may 
meet with a vitreous or semi-vitreous base, in which the crystals 
are embedded ; in a third there may be a mass of very minute 
crystals, amongst which larger ones are porphyritically em- 
bedded.”* 
These varieties of structure are characteristic of similar 
rocks, whether dating from the earliest period of igneous action 
with which we are acquainted on our globe, or ejected in our 
own times from some volcanic vent. When the structure of a 
rock, whether crystalline, or vitreous, or otherwise, has been 
determined, we are furnished with a key which will help us to 
open the secrets of the mode of formation, and the origin of the 
rock itself. 
Turning now to the minute examination of rock structure, 
let us look at some of the peculiarities of the igneous rocks. 
Before doing this, however, it may be well to have before us a 
rough outline of the general characters of these rocks, and from 
S. Allport, F.G.S. 
