OF ROCKS. 
95 
It is easy to see how these circumstances impart to the several 
rocks definite features, which characterize local districts, and make 
that variety of rock scenery remarkable in good pictures of different 
regions. 
At a distance of many miles the mountain limestone scars of York- 
shire distinguish themselves by form as well as other marks from the 
brows of gritstone and the peaks of slate, and all these are differenced 
by the observing eye from colonnades of pyrogenous rock. 
Geographical relation of Joints . — We may next compare the joint- 
planes to the general surface and polar points of the globe. The re- 
lation of joints to horizontal planes is through all the secondary and 
tertiary strata (uninfluenced by local pyrogenous rocks) accidental ; 
being in fact dependent on the inclination of the surfaces of strati- 
fication. But in the slate districts, the question is more difficult. If 
it be correct to consider the ‘spires’ of the Ribblesdale slate surfaces 
of stratification, (see p. 8,) the constant vertically of the N. and S. 
joints may perhaps be really dependent on their crossing the strata 
nearly at right angles to the dip, while the variously oblique and 
horizontal joints seem reducible to no exact relation either to the 
surface, the stratification, or the cleavage. 
But there remains a very interesting subject of research, on which 
it is possible to acquire precise information, viz. the relation of the 
direction of the joints, as measured on a horizontal plane, to the polar 
points of the globe. It has already been stated that in the slate tracts 
of Craven vertical north and south joints are eminently characteristic 
constituting the ends of the ‘ spire.’ 
In old red sandstone at Brough, N. 15° E. 
In fifteen observations of long joints in the lower mountain lime- 
stone series I found the directions as under. 
