92 
SYMMETRICAL STRUCTURES 
molecular constitution, the joints vary in number, regularity, direction, 
and other characters. 
In pyrogenous rocks, it generally happens that the planes of their 
joints are perpendicular to the bounding surfaces, (where the cooling 
commenced,) so that in the stratiform basalt and greenstone of Teesdale, 
their intersections are vertical, but in the great basaltic dyke of Cockfield, 
horizontal. It also happens that the joint-planes are not in general 
continuous through one another, but from each intersection a new 
plane arises, thus breaking the whole rock into prismatic masses, and 
leaving no doubt in the observer’s mind of the contemporaneity of all 
the divisions. There are nevertheless, in certain districts, a few master- 
fissures observable. This prismatic structure is beautifully exhibited at 
the High force in Teesdale, where the basalt has impressed the same forms 
upon the subjacent shale, so exactly, that it was only on a second visit to 
that romantic spot that I was undeceived as to the nature of the phe- 
nomena. At some distance above and below, this structure entirely va- 
nishes. (See Diag. No. 12, and page 79 J 
In the stratified rocks of Yorkshire we notice, but can hardly des- 
cribe, peculiar characters of the joint structure corresponding to each 
different rock. Throughout the slate and carboniferous tracts the 
laminated argillaceous rocks are most minutely and perfectly jointed, 
the thick massive beds of sandstone irregularly cracked rather than 
fissured, the limestones locally cracked, always jointed, and fissured. In 
the magnesian limestone, the laminated upper beds are regularly and 
symmetrically jointed, but the thick yellow limestone beds, exhibit little 
of this character. The laminated lias clays are as regularly jointed as 
some of the coal shales, but the more homogeneous clays of the oolitic 
series above, shew this character feebly ; the limestones are jointed and 
fissured, and the sandstones irregularly cracked and fissured. In all 
systems of rocks, conglomerates of all kinds have little symmetry in 
their divisional planes, but are traversed by great fissures : coarse sand- 
stones are more irregularly divided than finer rocks, and laminated 
shales lose on becoming sandy or calcareous most of their minute 
