302 
GEOLOGY OF THE FOURTH DISTRICT. 
produced by corals, and often much crystallized, being in a condition to exhibit less of these 
divisions on desiccation, is nevertheless always strongly characterized by a jointed structure. 
Slates, also, where the laminar structure is well defined, have always this jointed structure in 
great perfection ; while in those where the slaty cleavage is not observed, the joints are like¬ 
wise very obscure, or but imperfectly developed. Although there may be no sufficient reasons 
to induce the belief that one is dependent upon the other, still, in rocks of similar composition, 
where one is absent, the other is likewise. 
In many instances we find a fossil perfectly divided by a joint, the respective portions re¬ 
maining on either side of the fissure. Numerous instances of this kind may be seen in the 
plane of a single joint in the fossiliferous portions of the Chemung group. Had these joints 
been due to any thing like the cracks in ordinary clays, or those produced during desiccation, 
it is hardly possible that these fossils would have been divided in this regular manner. 
This structure is fully illustrated by the following woodcuts and descriptions. These 
sketches could have been greatly multiplied, but these only have been selected from a nu¬ 
merous collection, particularly in the higher rocks. 
Jointed Structure of the Limestones. 
The direction of these joints in the limestone is for the most part very uniform ; still, how¬ 
ever, there are exceptions. Where the rock is exposed upon the surface, these lines of joints 
are very manifest, and from weathering, often present wide fissures ; these, in favorable situa¬ 
tions, become so much worn beneath the surface as to be cavernous. They influence the 
thin soil upon the surface by draining the water into the fissures of the rock, often leaving the 
surface dry. 
In the lower part of the Niagara limestone these are prominent characters, and in the quar¬ 
ries near Lockport, are of the greatest importance in facilitating the removal of the stone. 
The central portion of the mass, which is concretionary and irregularly bedded, does not 
present the well-defined joints of the lower part; indeed, in some places they appear to be 
nearly or entirely wanting. Again in the upper part, which is thinly bedded, though curved 
and apparently contorted, the joints are strongly defined, and together with the thinly bedded 
structure, enable the workmen to quarry the rock without the aid of blasting. The sketch 
on page 94 is a natural face of the rock in the direction of a joint. 
In the limestones of the Helderberg division, this structure is more strongly manifested than 
in the Niagara limestone. The joints on the surface often appear as broad fissures, leading 
to caverns below, and the sinkholes observed are produced by the widening of these fissures. 
Where this rock is wrought for quarry stone, the same structure is strongly manifested, the 
lines of division often penetrating the entire rock. The woodcut below is an illustration of a 
quarry worked on the line of outcrop, and shows the facility offered in removing the rock. 
