It enabled me to locate not only the right horizon but also a horizon in the underlying heavy Comanchean limestone, which is water-bearing, and which must be avoided to prevent serious injury to the underground operations. I need not add that the information obtained was of real practical value in this case. PLATE I Plate I. Forms of frost cracks seen on the exposed flat bedding plane of a block of Dakota sandstone in a ravine a few miles southwest of Minnekahta, South Dakota. As sketched in the field. Natural size. [Illustration: PLATE I] PLATE II Plate II. Photographs of fossil imprints of ice crystals on flags of the Eagle Ford. The upper rock shown in the Plate is from the south side of Cuesta Blanca in Brewster County, Texas, and shows casts of crystals which represent fillings of sandy mud projecting slightly down into an underlying bed of more argillaceous material. The lower part of the figure shows a similarly marked flag from the same formation at a point about five miles north of the old Boquillas postoffice near Tornillo Creek, east of the Chisos Mountains, in Texas. Here are seen the original grooves made by the ice on a layer of muddy material later buried. Slightly reduced. [Illustration: PLATE II] PLATE III Plate III. Photograph of fossil casts of a close tangle of ice crystals seen in a stony calcareous layer in the Eagle Ford shale in Walnut Creek, about eight miles north of Austin, Texas. This tangle is closer than any of the recent ice crystal marks figured here, but equally closely grown crystals have been seen by the writer on frozen mud in Illinois. Natural size. Compare with Plate VIII. [Illustration: PLATE III] PLATE IV Plate IV. Photograph of fossil casts of ice crystals seen on some stony flags in the upper part of the Eagle Ford at Fossil Knobs, about two miles northwest of the Chisos Mine in Brewster County, Texas. These may be characterized as relatively short and scattered. This shows ridges projecting into the grooves formed by ice crystals on the surface of a muddy layer originally underlying the layer photographed. Slightly reduced. [Illustration: PLATE IV] PLATE V Plate V. Photograph of fossil casts of ice crystals seen on the under side of flaggy layer of calcareous sandy rock in the upper part of the Eagle Ford at Fossil Knobs, about two miles northwest from the Chisos Mining Company's property, Brewster County, Texas. It will be seen that some of the crystals are gently curved. Similar curving crystals are also seen in the figures showing recent growths at Rock Island, Illinois. Compare with Plate IX. Slightly reduced. [Illustration: PLATE V] PLATE VI Plate VI. Photograph of a thin flag of sandy limestone from the Eagle Ford at Fossil Knobs in Brewster County, showing molds left by ice crystals. [Illustration: PLATE VI] PLATE VII