In vol. xv. (New Series) of the American Journal of Science,[203] an extract is given from a letter describing the ‘Ice Spring’ in the Rocky Mountains, which the mountaineers consider to be one of the curiosities of the great trail from the States to Oregon and California.  It is situated in a low marshy ‘swale’ to the right of the Sweetwater river, and about forty miles from the South Pass.  The ground is filled with springs; and about 18 inches below the turf lies a smooth and horizontal sheet of ice, which remains the year round, protected by the soil and grass above it.  On July 12th, 1849, it was from 2 to 4 inches thick; but one of the guides stated that he had seen it a foot deep.  It was perfectly clear, and disposed in hexagonal prisms, separating readily at the natural joints.  The ice had a slightly saline taste,[204] the ground above it being impregnated with salt, and the water near tasting of sulphur.  The upper surface of the stratum of ice was perfectly smooth.

In Poggendorff’s Annalen (1841, Erganzsband, 517-19,—­Boue, an old offender in that way, says 1842) there is an account of ice being found in the Westerwald, near the village of Frickhofen at the foot of the Dornburg, among basaltic debris about 500 feet above the sea.[205] Commencing at a depth of 2 feet below the surface, the ice reaches from 20 to 22 feet farther down, where the loose stones give place to dry sand.  The ice is in thin layers on the stones, and is deposited in the form of clear and regular hexagonal crystals.  The lateral extent through which this phenomenon obtains is from 40 to 50 feet each way, and is greater in winter than in summer.  As in other cases that have been noticed in basaltic debris, the snow which falls upon the surface here is speedily melted.  The Allgemeine Zeitung (1840, No. 309), from which the account in Poggendorff is taken, suggested that the melted snow-water which would thus run down among the interstices would readily freeze below the surface, while the heavy cold air of winter would be stored up at the lower levels, and the poor conducting powers of basaltic rock[206] would favour its permanence through the summer.  The temperature of the cold current which was perceptible in the parts of the mass of debris where the ice existed was 1 deg.  R. (34 deg..25 F.).  Nothing but a few lichens grow on the surface of the debris.

These are, I think, all the references I have met with to the prismatic structure of subterranean ice.  But there is an interesting account in Poggendorff ’s Annalen,[207] by a private teacher in Jena, of the crystalline appearance of ice under slow thaw near that town.  In the winter of 1840, the Saale was frozen, and the ice remained unbroken till the middle of January, when the thermometer rose suddenly, and the river in consequence overflowed the lower grounds, and carried large masses of ice on to the fields, where it was left when the water subsided.  On the 20th of January the thermometer fell again, and remained below the freezing point till the 12th of February:  some of the ice did not disappear till the following month.