STEREOGRAM OF THE HENRY MOUNTAINS. For the double purpose of mapping and studying the mountains a model in relief was constructed, in which great pains was taken to give all the principal features their proper altitudes and proportions. This model was photographed, and has been reproduced by the heliotype process in Plate III, at the end of the volume. After it had been completed, a second model was constructed by adding to the surface of the first. Wherever the Blue Gate sandstone appeared in the original model no addition was made. Where the Tununk sandstone appeared there was added an amount equivalent to the combined thickness of the Blue Gate sandstone and the Blue Gate shale; and in general, enough was added in every part to bring the surface up to the summit of the Blue Gate sandstone. In this way a restoration was made of the form of that sandstone previous to its erosion. It is not to be understood that the mountains ever possessed this form; for when the surface of the Blue Gate sandstone was unbroken by erosion it was unbroken only because it was covered by other strata, which while they shielded it were themselves eroded. If the sandstone had been indestructible, this is the form which would have been developed by the washing away of all the overlying beds; and in this form are embodied the arches and domes which were impressed upon the sandstone by the upswelling of the laccolites. The model became a stereogram of the displacements of the Henry Mountains, and a photograph from the stereogram appears in Plate IV. It will be observed that over the central district of Mount Ellen, where by reason of the peculiar sculpture of the rock its structure was concealed, the restoration of the sandstone was not carried; it seemed better to represent our lack of knowledge by a blank than to bridge over the interval by the aid of the imagination. If the reader will study the plate, he will find that it expresses a great body of the phenomena which have been described in this chapter. The simplicity of the Ellsworth and Holmes arches is contrasted with the complexity of the others; the greatness of the Hillers and Pennell domes with the smallness of those which lie upon their flanks. One point that is especially striking is the relation between the upper and lower domes of the Ellen cluster. The upper, which give rise to all the conspicuous features of the mountain flanks, are comparatively small, while the lower, which might almost be overlooked in a rapid examination of the mountain, are comparatively large and constitute the great mass of the uplift. The smaller laccolites, because they are the upper, have been denuded of their covers, and in virtue of their hardness stand forth salient. The larger, because they are the lower, have not been laid bare, and the comparatively feeble resistance which their covers have opposed to erosion has impressed their forms but slightly on the topography.