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POPULAR SCIENCE REVIEW. 
cals, by Professor Hull, Mr. Samuel Allport — to whose kindness 
in allowing me to inspect his valuable collection of rock sections 
I am much indebted — Professor Greikie, and one or two other 
eminent geologists, who have recognised the value of the micro- 
scope in their investigations. The papers to which I allude 
being merely descriptive of certain rocks, mostly igneous, pre- 
supposes a certain amount of knowledge of the subject on the 
part of the reader. The Hermans are ahead of us in these 
inquiries, and they have recently published one or two im- 
portant works on the subject.* 
In the present paper I purpose showing, as far as the little 
knowledge I have been able to acquire will enable me, how 
much may be done with the microscope in unravelling the 
mysteries of rock structure, trusting that other observers may 
be stimulated to enter upon this neglected but wide and most 
fascinating field of study. 
To the diligent student there is no rock that will not, on 
sufficient examination, tell something of its history. The 
coarsest rocks reveal part of their history to the unaided eye, 
and the most trivial inspection tells us at once that a conglome- 
rate or a grit must have assumed its present condition through 
the agency of water ; it is the same with most sandstones, clays, 
and shales. The life-history, too, of such rocks, as well as of 
the generality of limestones, is told in a similar manner by their 
fossil contents, which are usually large enough to be seen with- 
out any instrumental aid. But when you would inquire further, 
when you would learn the previous history of the component 
parts of such rocks, the eye alone can learn but little. Here, 
then, comes in the use of the microscope, which will find a 
history written in the minutest chip or grain taken from any 
given rock. By its means you may also learn whether a rock 
whose structure is too minute to be understood without it, is to 
be classed amongst the igneous or the aqueous series. You 
may learn whether such a rock owes its origin to igneous 
fluidity or to sublimation ; whether it has been deposited as a 
sediment from water, or crystallised from that fluid at a more 
or less high temperature; whether it has been subjected to all 
three agencies, igneous fluidity, sublimation, and heated water, 
at once or successively ; and even something may be learnt as 
to the amount of heat and pressure it must have undergone in 
the course of its formation. Again, the microscope will serve 
* u Untersuchungen iiber die mikroscopische Zusammensetzung der 
Basaltgesteine,” by F. Z'rkel; Bonn, 1870. “Mikroscopische Beschaffenheit 
der Mineralien und Gesteine,” by F. Zirkel, 1873. “Mikroscopische Phy- 
siographie der petrographisch wichtigen Mineralien,” by H. Rosenbusch; 
Stuttgart, 1873. 
