* 
Lithological Survey of Schehallien. 357 
exception, to be probable) that the column remains the same 
through its whole length, we can compare the real attraction 
with that assigned to it in Dr. Hutton’s calculation. The 
attraction of any column computed on his hypothesis being 
divided by 2.5, and multiplied by the true specific gravity, will 
give the real attraction, or effect in disturbing the plumb-line. 
It is on this principle that our correction is formed, though 
simplifications occurred that very much diminished the labour 
of the computation, the nature of the rock leading us in the 
end to distinguish only two differences of specific gravity; and 
the ingenious deductions of Dr. Hutton, together with the 
excellent order that prevails in his computation, having made 
it easy to follow a route which he had cleared of all its great- 
est difficulties. 
However, as it was impossible to determine before hand 
how much the specific gravity of these rocks might differ, it 
was necessary to conduct the survey so that every individual 
column, had it been necessary, might have had its specific 
gravity defined. For this purpose the mountain was traversed 
in various directions, and the points at which a transition was 
made from rocks of one character to those of another were 
carefully noted, and their position ascertained. In selecting 
the specimens which were to represent the rocks of the seve- 
ral districts into which the mountain thus became divided, 
attention was paid both to the prevailing stone, and to that 
which was least common, in order that we might, if possible, 
get possession both of the mean and the extremes. Tins was 
in general the principle that guided our choice of specimens, 
but the application of it in detail to particular instances does 
not admit of being explained. The reasons in every such 
1 
