Page:The Journal of geology (IA journalofgeology11893univers).pdf/387

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 1:4000. 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.^[1]

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-