The ice on the bottom of caverns of course takes its shape from the form and angles of the floor of the caves. If the bottom is level or nearly so, the ice lies on it as a sheet or floor. If the bottom of the cave is sloping, the ice follows the angles of the slope, forming an ice slope or ice wall, and sometimes becoming nearly or quite vertical. These ice slopes distantly resemble the portions of glaciers called an ice fall, with the great difference, however, that there are no crevasses, not even tiny ones. Occasionally, slabs of ice are found reposing in a fractured sheet over a solid ice floor. This means that a lake has formed on this spot in the spring, frozen over, and then run off, leaving its frozen surface in broken pieces on top of the under ice. Another kind of frozen water is the hoar frost which forms on the rock roofs and walls. This is not at all rare. It is an open question whether this is not the same thing as that which has been described as subterranean snow.[29] I found myself in Dóbsina a small sheet of what to look and touch was snow. I wrote of this as snow in my first paper about glacières[30], but I am of the opinion now that it was the hoar frost detached from the roof and not genuine snow. [29] See Part III.: Ziegenloch, page 247; Creux de Souci, page 207. [30] _Ice Caves and the Causes of Subterranean Ice_, November, 1896, and March, 1897. At Dóbsina, also, I noticed that the ice of the ice wall of the Korridor assumed a stratified or laminated form. Mr. John F. Lewis of Philadelphia suggested to me that this was probably due to a precipitation of the hoar frost from the roof, and I think his explanation is correct. The hoar frost forming at a certain degree of cold, would doubtless be precipitated at a rise of temperature, and would then act much as do the different layers of snow in the upper portion of glaciers.[31] It would consolidate gradually, layer over layer, and form strata, producing the banded or laminated structure visible in the vertical ice of the Dóbsina Korridor. [31] Whymper: _Scrambles amongst the Alps_, 1871, page 426. The ice in caverns is sometimes found with a structure which is, I believe, of rare occurrence above ground. This is when it takes the shape known as prismatic ice, which means that if a lump is broken from a column or icicle, the fracture will show regular prisms. This phenomenon is not as yet satisfactorily accounted for; the only thing certain about it is, that it does not occur in ice of recent formation. From my own observations, I should say that ice became prismatic at the end of summer; at least I have always found it in August or September rather than in June or July.[32] [32] See Part IV.: Browne, page 289; Lohmann, page 303.