595 
of Edinburgh, Session 1877-78. 
If a<p, similar reasoning shows that the value is 
f r2 ~( p ~ jT) 
a(r 2 + (p - jJ - 2 r[p - ^)cos a) 
These values agree, as they should do, when a —p. 
Monday, ls£ April 1878. 
Sir WILLIAM THOMSON, President, in the Chair. 
The following communications were read 
1. Chapters on the Mineralogy of Scotland. Chapter IY. 
Augite, Hornblende, and Serpentineous Change. By 
Professor Heddle. 
(Abstract.) 
A. couple of months ago I had the honour of submitting to the 
Society a speculation upon the metamorphism of a particular rock 
mass. To-night I again return to metamorphism, submitting, how- 
ever, not a speculation, but the closely elaborated process of the 
change which has affected another rock. 
It is perhaps natural that the attention of one who approaches 
geology from the chemical and mineralogical sides should be imme- 
diately directed to those rocks which are either aggregates of simple 
minerals, or which are the products of changes effected upon . simple 
minerals ; natural also that consideration should be first given to 
those in which that change has been more immediately chemical 
than physical. 
To no rock mass does this apply more directly than to serpentine. 
In my wanderings I have visited and closely observed the relation- 
ship of — with a single exception — every bit of this rock which is to 
be found between Unst and Tyree on the one hand, — Harris and 
the Black Dog Pock on the other. 
In my analyses of a number of ill-defined minerals, generally 
believed to be products of the alteration of augite and hornblende, 
