OF ROCKS. 
93 
jointing, but in the latter case contain more secret or dry cracks. Very 
thick and uniform masses of limestone exhibit prismatic structures as in 
pyrogenous rocks. 
One more relation of joints to the nature of the rock is to be 
noticed. The width between their faces varies to a considerable ex- 
tent, so that the sum of the spaces which they leave in the rock is 
greatest in limestone, least in fine clays and coal, and of an intermediate 
amount in gritstone and cherty beds. This fact is best seen by con- 
templating some of those great fissures which pass through all the 
beds of a mountain limestone district ; and widen to the extent of 
some inches or a foot in the limestone, but contract to a narrow chink 
in the plates, 
Another relation to rocks is observable. Joints are most numerous 
and symmetrical in the older rocks. Compare for instance the laminated 
grauwacke of Westmoreland with the plates of the limestone series of 
Yorkshire ; these with the laminated lias of Whitby ; the lias with the 
Oxford clay or Kimmeridge shales; and generally the argillaceous 
terms of any series below the chalk with those above it. Again, com- 
pare the thin bedded carboniferous and magnesian limestones; either 
of these with the lias limestones ; any of these with the forest marble 
or Wealden groups; or let the flagstone below the millstone grit be 
compared with that above it, and both with the flagstones of the oolitic 
or Wealden formations, 
It may occur as an objection to this conclusion that gneiss and mica 
schist have less of this structure than laminated sandstones : but to 
this two remarks apply : first, these rocks are almost always very 
coarsely grained ; and secondly, where, as in the country about Dalna- 
eardoch, they are of finer grain, the jointed structure appears in great 
perfection. In these old rocks the variation of joints in the ratio of 
the lamination is well seen by comparing the black limestone of Glen 
Tilt, with the thin bedded rock of Loch Earn— dhe former being 
fissured, the latter minutely and regularly jointed. 
