A somewhat different kind of ice sheet was observed on Mount Etna. Sir Charles Lyell[19] speaks of it as a "glacier preserved by a covering of lava." He says Signor Mario Gemmellaro satisfied himself that nothing but a flowing of lava over snow could account for the position of the glacier. Ice sheets somewhat similar to these are reported from Tierra del Fuego,[20] and probably also such sheets occur in Iceland; and enormous heaps of ice covered with sand are found on Mount Chimborazo.[21] On the northwestern coast of Greenland, glaciers, whose flow has stopped, were observed buried under an accumulation of moss and grass.[22] [19] _Principles of Geology_, 11th Edition, Chap. XXVI. [20] See Part III: page 190. [21] See Part III: page 189. [22] See Part III: page 165. _Taluses and Boulder Heaps._--Taluses and broken debris, and in general boulder heaps of all sorts, have interstices and openings between the boulders, and in these it occasionally happens that ice is found. This is most common among the taluses at the base of cliffs, but in some cases ice is found among broken rocks on the sides of gently sloping hills, or even on the plateaus of their summits. Sometimes the ice and snow on the bottom of rock gorges all melts away, while further down, in the hollows of the boulders forming the floor, ice still remains. The rocks of which these ice bearing taluses are formed are generally gneiss, granite, limestone, sandstone, basalt or porphyry. Among such boulder taluses the phenomenon designated as _Windröhren_ or _Ventarolen_, that is, windholes, is frequently found. Sometimes the air among such boulder formations is quiet, but as a general thing draughts pour out at the lower openings during the hot months, and blow into them during the cold ones. _Freezing Wells, Mines and Tunnels._--Subterranean ice is also found in certain places in connection with man's handiwork. In a few wells in the United States, the temperature in winter becomes abnormally low, and for four or five months these wells freeze up and become useless. A case of a freezing well was recently observed near la Ferté Milon in Central France.[23] Ice is reported also as forming in various mines in Europe, Asia and America; in fact, it is not an uncommon occurrence. Occasionally, also, ice forms in tunnels. [23] See Part I.: pages 74, 79, 89. Part III.: page 206. _Cold Caves._--Caves with abnormally low temperatures may be divided into two classes. First, caves where the temperatures are lower than the normal, without becoming low enough for ice to form; and second, caves where the temperatures sink so low, that ice forms.