
PREPARATION OF TRANSPARENT SECTIONS OF ROCKS, ETE: 103 

polariscope, and having prepared sections of calcite and other 
minerals, I had a box made to hold these, and when first preparing 
sections of rock the natural thing was to fasten the rocks on these 
pieces of glass. The result is that I have now a very large 
collection of sections of rock, all mounted on squares of glass of 
this size. 
With these few preliminary remarks I will begin by giving an 
account of the method I have adopted in preparing sections of 
rock, 
Of course everything had to be learnt, and there were then 
none of the facilities you have now. 
The most obvious method now would be to use a slitting 
machine, but I commenced and afterwards continued to adopt a 
method which would not be used now, but which led to good 
results. I found that if you had specimens such as you would 
get out of a museum, it would be very desirable to saw thin 
portions off, but nearly all the specimens that I have studied have 
been collected zz s¢/#. Of course there are obviously great 
advantages in that, because you are then acquainted with the 
conditions under which the specimens are found, and you can 
break off thin portions. I always used to break them at right 
angles to the stratification, and in the case of rocks with slaty 
cleavage, one portion in the line of the dip and. another in the 
line of the strike, reserving larger pieces for hand specimens. 
Having then collected specimens in that manner, the next thing 
is to deal with them in such a way as ultimately to give you a thin 
section. A deal of the work I did myself, but afterwards I found 
it very convenient to get much done by glass cutters. I had them 
thus ground into portions such as the specimens I have here, say 
about one inch square and 2 inch thick. Then, having got a 
portion of rock like this, the next point is to finish off one surface, 
so as to have it as perfect as possible. You can well understand 
that it should be absolutely flat,—a perfect plane,—because many 
sections of limestone must not be more than tooo In thickness. 
If not quite flat in grinding them down, ultimately you are liable 
to grind one portion away whilst the other is too thick: The 
method adopted to obtain a perfectly plane surface on which to 
rub these sections was as follows : I had a sort of flagstone, about 
two feet square, fixed on a table in the yard,—it was a sawn flag, 
tolerably level; then I had two portions of little flags, about 
fourteen inches square, of good quality, such as you obtain in this 
neighbourhood ; then the first consideration was to rub these two 
flags with emery backwards and forwards on the large flag till 
perfectly smooth. You might get them both a little convex, 
perhaps, but both alike. The next thing to do is to rub these two 
stones together, and the stones being of the same hardness and 


