Wherever an incision was made in this sheet of ice, the prisms snapped off at the depth of an inch, and could be mowed down like corn by means of a stout knife.

Although they broke naturally at this constant depth, and left a surface of limpid ice without any signs of external or internal division, still the laminae obtained by chiselling this lower surface carefully, broke up regularly into the shapes to be expected in sections of prisms cut at right angles to the axis.

The roughness of my instruments made it impossible to discover how far this extended, and whether it ceased to be the case at any given depth in the ice.
The sea of ice on the floor was in a very wet state at the surface, being at a lower level than the stones on to which the rain from the open hole fell; and here the prismatic structure was not apparent to the eye, nor do I know whether it existed at all.

In the Glaciere of La Genolliere I carried a large block of perfectly prismatic ice into the outer cave, where it was exposed to the free currents of air passing from the pit of entrance to the hole newly opened by the falling in of the ground; and, two days after, the external lines were scarcely perceptible, while on the occasion of our third visit I found that they had entirely disappeared, and the whole block was rapidly following their example.

This disappearance of the surface-lines under the action of atmospheric thaw is probably the same thing as their absence when the flooring of ice is thinly covered with water.