701 
rinally, to render this introductory preface as complete as possible, I will briefly 
describe the method of preparing rock-sections. 
Sorby * (one of the fathers of petrography) prepared his sections by grinding a 
flake of the rock and polishing it on one side.f The polished surface is nest cemented 
with Canada balsam to a piece of plate glass, which latter forms a sort of handle, 
as well as iusuring more or loss parallelism between the two faces of the polished 
film. The best grinding surface is plate glass with a sprinkling of emery powder 
(No. 40) ; and for polishing, I always use flour emery, on another and similar glass 
plate. The powders are kept fairly moist with water. The rough side of the mounted 
flake is then ground down till a mere film of rock remains adhering to the glass. 
That film is the “ section.” When properly ground it is in most cases perfectly 
transparent — hornblende transmitting green light ; quartz, white light ; and felspars 
being more or less transparent, according to the changes that the rock may have 
undergone. 
Different students have, of course, different methods of working. I' use a 
lapidary’s wheel, which facilitates the work ; but the simplicity of the other apparatus 
commends itself to a frugal mind. 
Having then obtained a film of rock containing various contiguous minerals, the 
thickness of which may vary between '006 inch to ‘001 inch, we proceed to mount it in 
Canada balsam, and then to examine it according to the methods detailed in the above 
sketch. In this way, a rock-section, when carefully prepared and examined, reveals to 
a trained eye different contiguous mineral films. The structure and texture of the 
aggregate is first noted ; next the minerals are examined for their optical properties, 
which are compared with the tabulated optical constants of rock-forming minerals, and 
thus identified. 
Lastly, under a high power, 4-iuch or i-inch objective, the gases, liquids, and 
solids included in the different minerals arc observed. Under such a power quartz 
reveals vast masses of gas-bubbles and liquids with little oscillating bubbles. These 
bubbles go on travelling round the walls of their tiny prisons apparently for ever ; and 
in one section, '001 inch thick, literally scores of these enclosures can be seen betvyeen 
the two polished surfaces of the section. In one of Tuess’s sections under '001 inch 
thick, belonging to Mr. Jack, I have counted over ten almost superimposed, yet never 
touclung each other, and with room for a score or so more, in each of which the little 
bubble flew round. J In this particular section there were over 1,000 enclosures in every 
cube whose edge measured '001 inch, and this was by no means a quartz rich in 
enclosures. Quartz is not the only mineral exhibiting such phenomena. 
In concluding these few Notes, introductory to the science of petrology and 
petrography, I will leave the general reader to form what judgment he can on the 
micro'scopic structure of some of our Queensland rocks, asking pardon from students 
of geological science for my temerity in writing such a bald introduction to so vast a 
subject. 
A. W. CLAEKB. 
Charters Towers, 26th June, 1892. 
* H. C. Sorby, F.R.S., late Pres. Geol. Society. „ , u i,- 
t Mr. W. H. Rands, F.G.S., Assistant Government Geologist, obtains excellent flakes Iw Smashing, 
with a vigorous blow from a sledge hammer, a large lump of the rock ; amongst the ddbris, Mr. Rands states 
f.hafc ffenerallv S6V6r8.1 suitfl-blc flulcfts will be found* , 
t Mr. Jack informs me that these sections have been in his possession for more than sixteen years ; 
and yet they still show the oscillation. 
