PREPARATION OF TRANSPARENT SECTIONS OF ROCKS HC. 135 

well with turpentine so that it might penetrate into the pores of the 
rock, and then covered it over with Canada balsam, and kept it 
hot inside the fender in the room. 
The result was that the balsam penetrated into the loose material, 
and ultimately got hardened. The balsam thus supplied artificially 
what Nature had failed to supply, in not having hardened it 
sufficiently by infiltered quartz. 
Then one could proceed to work. But when you have got 
some little distance down, it may be as well to repeat the process 
once or more. 
This method is necessary for rocks of the softest description. 
It is necessarily tedious, but very important results come from the 
study of the rocks most difficult to deal with. 
You have then the mica-schist not at all broken up, but the 
weak points and the planes of discontinuity filled with hard Canada 
balsam. It is, in fact, thoroughly hard throughout, and you can 
rub it down and leave the section of the thickness that you desire. 
You would hardly believe what awkward things some of these 
mica-schists are. They are not only foliated with alternations of 
mica and quartz, but you have what had been originally flat planes 
of foliation all crumpled up in the most complicated manner, and 
avast number of joints; another set of planes of discontinuity 
crossing all the others, so that you have lines of weakness in every 
direction that you could wo¢ desire; but with a little care and 
management I succeeded in making sections of these rocks 
that left nothing to be desired ; and I do not know that I was ever 
able to observe facts of more interest than in the study of some 
of these rocks. 
One of them was the mica-schist in the neighbourhood of 
Dunkeld, and I was able to unravel a number of most interesting 
problems. I was enabled to ascertain that a considerable amount 
of the quartz was bona-fide grains of sand,—a most important 
point on the origin of mica-schist, for it is a most complete proof 
that they were originally rocks containing grains of sand, as well 
worn as you could get them in the Thames, over white quartz 
subsequently crystallised in perfect optical continuity. 
There you have the history of the original material and the 
history of the chemical changes that took place, and after this 
crystallisation had occurred, you have the subsequent history of 
the crumpling up and the formation of the joints. All these 
points were made out by the study of this most unpromising rock. 
I may say that in the case of some of these the trouble was 
thoroughly rewarded ; but whether it would reward another student 
is a different matter, because when the whole field was before me 
and these facts had never before been observed, it did not signify 
what trouble I was at to establish facts of this kind. But having 

