There are some discrepancies in this last paragraph which must be noted, for the reason that Trouillet's observations are so valuable. He does not mention having seen the vapors himself, in fact the production of these vapors seems only an inference. Nor is it easy to understand how the grotto could be "completely isolated from the external air" if the phenomenon lay "in the introduction of the dry air driven to the cave by the winds north and east." Dr. B. Schwalbe, in 1886, wrote that "all my observations point to the fact that the rock is the cooling factor in summer, and that the cold goes out from it." He says also that "when I saw for the first time the little cave of Roth, which was filled with fairly numerous ice formations, it was precisely the smallness of the volume of air and the strange appearance of the ice which made the simple cold air theory seen insufficient, nor could I later, by widening the theory and observing the localities from the basis of DeLuc's theory, accept it. It always seemed by all my observations that in the rock there must be a lasting source of cold. There must be a cause present, which prevents the rapid warming of the cave wall through the temperature of the ground, which also keeps the stone cool in summer and induces the main ice formation in the spring." He also hints that Mr. Lowe's compressed air theory may be the correct one. Dr. Schwalbe's work, _Über Eishöhlen und Eislöcher_, is one of the four or five most important contributions to glacière literature, and his opinion is entitled to great respect on account of his many observations. Professor Israel C. Russell wrote in 1890, about the ice beds on the Yukon: "It is thought by some observers, to be an inheritance from a former period of extreme cold; but under existing climatic conditions, when ice forms beneath a layer of moss, it is preserved during the short summer, and may increase as it does on the tundras, to an astonishing thickness."