8 
Quarterly Journal . 
older members, and any who have acquaintance with the 
microscope, will give a helping hand in this endeavour, in 
order to prove what can be done in the matter. I must 
say that, on looking back to the past, I think we may hope 
we have tided over a period of dulness and comparative 
difficulty, and that our prospects are somewhat brighter 
than they were a year ago. And trusting we shall each be 
able to add our quota in the future to the work which lies 
before the society, I beg to close the past year and open 
the incoming one to your welcome. 
Notes on the Examination of Thin Slices of Rocks under 
the Microscope by Means of Polarised Light By A. 
W. Ho witt, F.G.S. 
plead 30th November, 1876.] 
An examination of thin slices of rocks under the micro¬ 
scope by ordinary light reveals to us their general structure. 
We are in this way able to distinguish, in many cases, the 
outline of the constituent minerals, and in those cases in 
which these minerals are present in a crystallised shape, to 
judge from their outlines as to the probable form of the 
crystals themselves. The various colours with which certain 
minerals become transparent also afford means for arriving 
at conclusions concerning them. We can distinguish other 
constituents by their strongly-marked character, as, for 
instance, magnetite . With high powers we are enabled to 
perceive minute cavities containing portions of once glassy 
substances, or of fluids; or again, other cavities which may 
be regarded as having contained imprisoned gases. 
Much also may be ascertained as to the processes by which 
various rocks become metamorphosed or decomposed, and as 
to the minerals resulting from such changes. Thus, an 
examination by ordinary light yields much information of 
importance to the petrologist; but it is to the results obtained 
by the employment of polarised light that the greatest 
interest attaches, and it is in respect to these that I propose 
to note a few of the principal points of interest. 
