In 1897, Professor Russell says: "It is not probable that all the subsoil ice of northern regions has been formed in one way. Along the flood plains and on the deltas of rivers where layers of clear ice are interbedded with sheets of frozen gravel and vegetable matter, as is frequently the case, it seems evident that the growth of the deposit is due, in some instances, to the flooding of previously frozen layers, and the freezing and subsequent burial of the sediment thus added to their surfaces. When spring freshets spread out sheets of débris over the flood plain of a river, as frequently happens when streams in high latitudes flow northward, the previously frozen soil and the ice of ponds and swamps may be buried and indefinitely preserved." "There is still another process by which frozen subsoil may be formed in high latitudes: this is, the effects of the cold during the long winters are not counteracted by the heat during the short summers. Under the conditions now prevailing in northern Alaska, where the mean annual temperature is below 32° Fahrenheit, the frozen layer tends to increase the thickness from year to year just as the depth of frozen soil in more temperate latitudes may increase from month to month during the winter season. During the short northern summers, especially where the ground is moss covered, melting only extends a few inches below the surface." Mons. E. A. Martel, in 1892, wrote of the Creux-Percé: "I incline only, as in all the pits which narrow at the bottom (_avens à rétrécissement_) to attribute the chilling to the fall of the cold air of winter and to its non-renewal in summer." And at page 564 of _Les Abimes_ he says: "One knows that _evaporation_ is an active cause of cooling; therefore it is always cooler in caves near the drips of water. * * * I have positively noted this influence of evaporation near the drips of Tabourel (8° instead of 9.5°), of Dargilan, of the Cerna Jama, and in abysses with double mouths where there were strong draughts (Rabanel, Biau, Fosse-Mobile, etc.)." In December, 1897, Mons. Martel writes: "In short, the action of the winter's cold is the real cause accepted by * * * and recently confirmed by Fugger, Trouillet and Martel." And also: "It is probable that this influence [evaporation] is only real at rather high altitudes; this is at least what seems the result of the studies of the caves of Naye (1700 to 1900 meters) begun by Professor Dutoit."