35° JVfr. Playfair's Account of a 
of each variety ; and, lastly, the position of each relatively to 
the observatory. Fortunately the Geometrical Survey of the 
mountain, which had already determined not merely its su- 
perficial extent but its solidity, taken in combination, with 
some peculiarities in its structure, have enabled us to approx- 
imate, I hope with some tolerable exactness, to the knowledge 
of all these three circumstances. 
The plan, then, which we proposed to follow, and which was 
necessary to be pursued if our Lithological Survey was to corre- 
spond in any degree to the accuracy of the Geometrical Sur- 
vey, made under the direction of the Astronomer Royal, was to 
try to recognise the chain of stations which had been employed 
in that survey, in order that, by reference to those stations, 
we might be able to determine the points on the surface of the 
mountain from which our different specimens were collected. 
After these stations were discovered, we meant to traverse 
the mountain in various directions, and at any point where a 
specimen was taken, to determine our position by the bearings 
of any two of the stations that might be in sight, or by taking 
angles to three of them, or such other methods as occasion 
and circumstances might suggest. This was to be done 
where considerable variations in the external characters of 
the rocks gave reason to look for considerable variations of 
specific gravity. It was an operation that could not be necessary 
for every individual specimen, but it was one which must be 
necessary for determining the district over which stone of a 
particular character prevailed. In this part of the work we 
were to employ a theodolite, a sextant or a compass, accord- 
ing as more or less accuracy seemed requisite. 
As the marks of the stations were all effaced except some 
