of Edinburgh, Session 1876-77. 
393 
Monday , 1 $th April 1877. 
Professor KELL AND, Vice-President, in the Cliair. 
The following Communications were read: — 
1. On the Toothing of Un-round Discs which are intended 
to roll upon each other. By Edward Sang. 
(Abstract.) 
This paper contained an extension to discs of any shape what- 
ever of the principles explained in the “New General Theory of 
the Teeth of Wheels,” as applicable to circular discs. 
2. On the Mineralogy of Scotland. — Chapter II. 
By Professor Heddle. 
In this chapter Professor Heddle submitted the results of the 
analyses of Orthoclase from fifteen localities ; of Albite from four; 
of Oligoclase from eight ; of Labradorite from eleven ; of Andesiel 
from five ; of Anorthite from three ; and of Latrobite from two. 
The three last minerals being now, for the first time, recognised 
as British species. 
Dr Heddle also described a peculiar association of Orthoclase 
with Oligoclase, in crystals nearly of the form of the former, from 
certain localities ; he drew the conclusion from his researches that 
the above felspars are all well individualised, if not by physical, 
at least by chemical characters ; while they are probably more or 
less special to certain rocks. 
3. Least Hoots of Equations. By J. D. Hamilton Dickson. 
