319 
Lithological Survey of Schehallien. 
streams that it was easy, by the assistance of the hammer 
only, to procure specimens having all the conditions requisite 
for our purpose. 
Supposing, however, that all this was accomplished, it 
would be insufficient to determine the mean density of the 
mountain, unless the quantity of rock of each particular kind 
could also be estimated ; at least nearly. It was necessary, 
therefore, to know what proportion of the mountain consisted 
of one species of rock, and what of another, without which 
the average could not be determined. 
Had the mean density been the only thing wanted, it would 
have been sufficient to know the quantity of each variety of 
rock ; but in the search we were engaged in, it was necessary 
to know not only the quantity, but the position of each of these 
varieties, relatively to the observatories on the south and 
north faces of the mountain. This will be evident, when it is 
considered that it was the effect of each portion of the rock 
on the plumb-line in these observatories that was the thing to 
be found, and that this effect must vary not only with the 
density of the rock, but with its distance from the observatory, 
and its obliquity in respect of the meridian. The mean den- 
sity would therefore be insufficient for estimating the attraction 
of the mountain, could it be found ever so exactly ; and it is 
easy to shew, that while the mean density of a heterogeneous 
mass, and also its magnitude and figure remain the same, its 
attractive force at a given point may be greatly changed by a 
different distribution of the materials it consists of, relatively 
to that point. In order then to form an estimate of this 
attraction we must know, at least nearly, these three things, 
the varieties of rock composing the mountain ; the quantity 
