458 
The concluding Paper, contributed by Henry Clifton 
Sorby, Esq., F.R.S., of Sheffield, was 
ON IMPRESSED LIMESTONE PEBBLES, AS ILLUSTRATING A NEW 
PRINCIPLE IN CHEMICAL GEOLOGY. 
In certain conglomerates, consisting chiefly of pebbles of 
limestone, met with in various places on the Continent, and 
notably in the so-called Nogeljiue in Switzerland, a very 
curious and interesting phenomenon has been observed: one 
pebble penetrates into another, sometimes to such an extent 
that it nearly passes through it, without there being any 
apparent fracture. Usually, however, the impressions vary 
in depth, up to half-an-inch or more, and into them another 
pebble accurately fits; and when it is removed a hollow is 
left, as if it had been pressed into soft clay. Much atten- 
tion has been paid to this phenomenon in Germany and 
France, and various theories have been propounded, none of 
which in my opinion are satisfactory; but it has attracted so 
little attention in our own country, that I have met with 
very few of my countrymen who ever heard of the subject. 
On studying the German and French literature related to 
this question, references to which may be found in a Paper 
of mine published in Germany,* I soon became convinced 
that the facts could not be explained in a satisfactory man- 
ner on any known chemical or mechanical principles, and I 
therefore was induced to investigate the question with the 
view to ascertain whether it would lead to some new physical 
principle. One thing led to another, until I was led into a 
wide and almost unexplored branch of research, which in the 
course of time enabled me to establish the direct correlation 
of mechanical and chemical forces ; f in other terms, I 
* Neues Jahrhucli fiir Mineralogie, 1863, 801. 
t See Bakerian Lecture. Proceedings of Royal Society, vol. xii., v. 538. 
