356 Mr. Playfair's Account of a 
mountain, from about the level of the south observatory to 
the summit, consists of granular quartz, as no other stone is 
to be met with any where in that tract. This is the part above 
O, in the section of the mountain ; and the only question is, 
whether we shall consider the part in the interior of the 
mountain, immediately under this mass, as consisting of the 
same rock. When we first examined Schehallien, Lord. Webb 
Seymour and I were both of opinion that this was the most 
probable supposition. Since that time, however, having had 
an opportunity of examining some other of the Grampians 
where granular quartz is found at the summit, and where, 
nevertheless, it is certain that the same rock does not go down 
into the interior, there has appeared some reason to suspect 
that this maybe true of Schehallien. As the result of the cal- 
culus with regard to the earth's density is materially affected 
by these suppositions, I have given the result as I had first 
deduced it on the hypothesis, that the interior of the mountain 
is of granular quartz ; and also on the hypothesis that the 
quartz is confined to the upper part ; and that the lower part 
is entirely composed of mica and hornblend slate. 
In the computation which Dr. Hutton made of the attraction 
of Schehallien, he supposed its mass divided into 96 o vertical 
columns, and he computed the force with which each of these 
columns disturbed the direction of a plummet suspended in 
either observatory, supposing them all homogeneous and two 
and a half times as dense as water.* Now knowing from our 
survey, and the combination of geometrical with mineralogi- 
* cal observations, the specific gravity of each of these columns 
at the surface, and conceiving (what we have shown, with one 
* Phil. Trans. Vol. LX VIII. (1778), p. 6S9, &c. 
