94 SYMMETRICAL STRUCTURES 
From all that has been said we may collect these general results. 
There are peculiar types of jointed structures for rocks of different 
mineral composition ; in each of these rocks the type is variable in 
proportion to the lamination of its mass and the coarseness or fine- 
ness of its granular texture ; and the number and symmetry of the 
joints augments with the age of the rock. 
Relation of Joints to other Structures m Rochs . — -The geometrical 
characters of joints may be considered with reference to the surfaces 
of stratification and lamina? of composition (where these are not parallel 
to the former ;) and the general surface and polar points of the globe. 
First. The planes of the joints are in all the secondary and primary 
rocks independent of the lamina? of composition ; of this every obliquely 
laminated gritstone or oolite affords complete proof. 
Secondly. In limestones of all ages generally the planes of the 
joints, both great and small, are perpendicular or nearly so to the surfaces 
of stratification. The internal cracks in the laminated limestones shew 
amidst many variations the same general law. 
Thirdly. In laminated argillaceous secondary rocks the joints are 
chiefly perpendicular to the surfaces of stratification, but some instances 
occur, and they are not unfrequent in the plates of the mountain lime- 
stone, of joints oblique to these surfaces. In the primary argillaceous 
deposits the joints, perpendicular to what are presumed to be surfaces 
of deposition, are complicated with many others oblique to them in 
different directions, (as already exemplified in the description of the 
Craven slate rocks, p. p. 6, 7, 8.) 
Fourthly. In laminated sandstones the joints belong to the same 
general description as those in shales, but oblique joints are less frequent. 
In block gritstones the joint surfaces, though far less regular than in 
limestones, have the same vertical relation to the surfaces of stratification. 
