Abstract
It is commonly believed by geologists, as well as by coal miners, that the inner faces of the rocks which enclose intrusive masses were at one time in contact, and that each of these surfaces is the counterpart in form to the other, from which it has been severed by the forces to which the injection of the intrusive mass was due. In the case of a sill, for example, this belief implies that the rock floor below the sill and the roof above it were in unbroken contact at some time before the sill was intruded, and that the floor and the roof have been forced apart to a distance equal to the thickness of the intrusive mass.

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