JOINTED STRUCTURE. 
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Limestone quarry near Waterloo. 
In the impure limestones of the Salt group, these joints are often manifested in some degree 
of perfection, but they are more usually closed, and evident on the surface only from weather¬ 
ing. They are much more numerous than in the purer calcareous rocks, and separate the 
layers into small blocks. 
Jointed Structure of the Shales and Sandstones. 
In the incoherent strata of soft shales, like the Niagara shale and those of the Onondaga 
salt group, as well as in some of the softer shales of the higher groups, the jointed structure 
is but very imperfectly developed. In the softer portions of the salt group, this structure is 
scarcely more developed than in the recent clays; and almost the same may be said of the 
soft red marl and shale of the Medina sandstone, and the shale of the Niagara group. In 
some parts of the Hamilton group, and in the Cashaqua shale of the Portage group, joints 
are often scarcely perceptible. In the impure and less compact sandstones, the joints are 
imperfectly developed ; but in the purer and more compact strata of this nature, they are very 
perfect. In all parts of the higher groups, however, the joints are very perfectly developed, 
and very clearly and exactly defined. In the slaty shales, where they have been freshly 
exposed, the surfaces are scarcely separated, and of the most perfect smoothness and regu¬ 
larity. The surfaces are usually plane, but sometimes they are curved, as in the following 
woodcuts: 
