pitch, to be reliable, must be based on observations made over a considerable area. Wм. H. HOBBS. UNIVERSITY OF WISCONSIN, EXPLANATION OF PLATES. PLATE V.-Geological Map of portions of Sheffield, Mass., and Salisbury, Conn., based on the Sheffield and Cornwall sheets of the Topographical Map of the United States by the U. S. Geological Survey. Scale 1: 62,500. PLATE VI.-Series of Geological Sections to accompany Plate V. Their location is indicated on the map (Plate V.) Horizontal Scale: one inch equals one mile. Vertical Scale: one-eighth inch equals five hundred feet. PLATE VII.-FIG. 1. View showing the southern termination of one of the longitudinal undulations of the western schist anticlinal, as seen from the west. A, Southern limit of a ridge of Riga Schist (No. 10). B, Turnip Rock (Everett Schist). C, Barack M'Teth (Everett Schist). D, Knoll of Riga Schist. E, Tom's Hill in the distance. F, Ridge No. 6 (Riga Schist). The dotted and dashed line shows the approximate boundary between the Riga Schist and the Egremont Limestone, and the dotted line the approximate boundary between the Egremont Limestone and the Everett Schist. FIG. 2. View of schist ridges separated by belts of limestone at the southeast base of Tom's Hill near the railroad bridge. A, B, C, Schist ridges. D, Slope of Tom's Hill where a fourth schist belt is hidden in the trees. FIG. 3. Canaan Dolomite occupying the core of an anticlinal of Riga Schist at the south end of area No. 6. The view looks southeast. A, Outcrop of Riga Schist. B, Canaan Dolomite. C, Riga Schist. THE NEWTONVILLE SAND-PLAIN. 1. Introduction.-During the past year the writer has studied the Newtonville (Massachusetts) sand-plain under Professor Davis, of Harvard University, and after studying the deposit as it now exists, made a detailed map of the plain with its feeding esker. Then a model of the region was made in clay on the scale of I: 4000. This clay model was photographed, and is here reproduced in half-tone, in Fig. 1, Newtonville Sand-plain. The conditions of formation were then studied, and a second model constructed, showing a conjectural relation of deposits to the margin of the New England ice-sheet at the time of its formation. A photographic reproduction of this is given in Fig. 2, Ice-sheet Restored.' 2. Making the models.-The clay was built up in a solid mass to the greatest required height, and the details of form were then cut with graving tools. In making such models it is essential that the foundation for the clay should be firm and not liable to warp. A slate slab, or a piece of heavy plate glass answers the purpose well. While at work on the model it is important to keep the clay moist. So a box lined with rubber cloth should be provided, large enough to cover the clay without touching it, and an inner layer of muslin put in to hold the water. When the model is ready to have a plaster mold made, the edges should be trimmed square, tapering slightly up from the slate so that the mold will slip off easily, the surface oiled, boards placed an inch and a half from the four sides, and liquid plaster poured over it. After the plaster has set, it may be wedged up from the slate or glass, and lifted from the clay. Then the plaster negative should be carefully washed with a brush to remove all oil or clay stick 'Teachers or others who desire copies of models, photographs, or lantern slides can arrange for them by corresponding with the writer. ing to it, and when hardened with a thin solution of glue and dried it is ready for the taking of a paper positive. This papiermachè model is a close representation of the original clay. 3. A late glacial deposit.—A glance at the first model will show the typical form of these delta deposits, the esker like an arm, and the sand-plain like a hand with its finger lobes. The esker rises in height as it approaches the head of the plain. The top of the sand-plain slopes very gently downward from the head to the top of the lobes, but the front slopes of the lobes are much steeper, about twenty degrees. The sand and gravel are so little disturbed that the deposit cannot be pre-glacial. That the deposit was not made by marine or fluviatile action is shown by the three following considerations. First, an aqueous deposit of gravel, composed of fragments from the crystalline high-lands between two and three miles to the north, should have extended originally from its source outward; but the amount of denudation and transportation required to cut out these delta deposits from a continuous sheet extending across the Charles river to the crystalline highlands on the north, whence a large part of the fragments come, would be greater than the post-glacial denudation that has been measured elsewhere. Second, the delta front and the even sloping delta-plain imply standing water, and if this water level existed for so long a time as would be required to form such an extensive deposit, we should expect to find more evidence of its shore line in other localities than now exists. Third, the constructional forms, cusps, hollows, kettle-holes, at the head of the sand-plain are so marked that one cannot believe them to be the product of erosion. The kettle-holes and marshy depressions show that the plateau tops did not extend much farther than at present. The dwindling New England ice-sheet, whose existence is proved by other facts, supplies all the conditions necessary for the construction of such discontinuous deposits. The ice-sheet could not have advanced over the plain after its deposition, for the sand and gravel would have been easily carried away. There is no gullying of the sides of the sand-plain; therefore it was formed not so very long ago. But the gravel is evidently of glacial origin, being of angular and subangular pebbles, of great variety of material. The conclusion seems inevitable, therefore, that these deltas were formed during the retreat of the ice-sheet. 4. Stagnant, melting ice.-In the retreat of the ice-sheet there were parts at least which became too thin to move. As Professor Davis has said: During this time it must have melted irregularly, presenting a very uneven, ragged front, from which residual blocks may have been frequently isolated; and it must have endured longest in the valleys, where it was thickest, not only by reason of its greater depth, but also because its surface there, where motion had been fastest and longest maintained, must have been higher than on the hills-this being homologous with the variation in the thickness of a Swiss valley glacier from middle to sides." It seems to me that we must consider the change to have been gradual from a moving glacier to a stagnant one, and that there may have been times of renewed activity with a forward motion, even in the period of decline. Such forward motion may have had some influence in shifting the course of esker rivers and so have determined where the next sand-plain was to be built. So far as I know, this point has not been worked out in the field. Crevasses are formed as the ice moves, and change their position according to the tensions in the mass of the glacier. When the tension from motion has ceased, and the ice has become a diminishing, drift-covered mass, the condition represented in Fig. 2, we should not expect to find any crevasses remaining. They would either have been closed by the forward motion of the ice, or would have lost their distinctive character by the excessive melting of their sides, while the water would have washed detritus into them covering the underlying ice, and preventing it from melting as fast as that on either side. Such protection of the ice by detritus must have had great influence in determining the surface forms of the stagnant ice-sheet, as is shown in Professor Russell's account of the sand cones and the deposits in glacial lakelets. Bull. Geol. Soc. of Am., Vol. I., p. 196. |