355 
Lithological Survey of Schehallien . 
is found regularly crystallized. The crystals however are small 
and very thinly disseminated ; were they in more abundance 
this stone might be accounted a granite, as Professor Jameson 
has remarked of a stone of the same kind which he found in 
the island of Jura. 
From the vertical position of the strata we may infer with 
some probability, that the rock which breaks out any where 
at the surface, continues the same through the interior of the 
mountain, in the direction of a perpendicular plane, down to 
its base, or perhaps to an indefinite depth. The same stratum 
usually remains of the same nature to a great extent, when- 
ever we have an opportunity of examining it, whether in a 
horizontal or a perpendicular direction ; and it is not to be 
doubted that the same holds when no such opportunity occurs. 
When, therefore, we have on the surface a bed of mica slate, 
or of granular limestone, or of granular quartz, the probability is 
that the whole stratum all through the mountain is composed of 
the same materials. I must however confess, that I do not think 
that this probability is as strong with respect to granular quartz, 
as it is with respect to the micaceous rocks. These last com- 
pose the great mass of the Grampians ; and their characters, 
though not every where the same, change very slowly, and 
pass from one to another by imperceptible gradations. To 
the granular quartz this rule does not equally apply ; it is not 
general among the mountains of this tract ; it sometimes 
breaks off suddenly, and is replaced by rocks of a very diffe- 
rent nature. We cannot therefore with the same confidence 
assume the existence of this rock in intermediate points, when, 
we only see it in the extremes. This much, however, we 
know with certainty, that the whole of the upper part of the 
Z Z 2 
