[Illustration: Fig. 27A, B. Section and Plan of a portion of the Lower Grindelwald Glacier.]
Another effect is here beautifully shown, namely, the union of the transverse and marginal crevasses to form continuous fissures which stretch quite across the glacier. Fig. 27B will illustrate my meaning, though very imperfectly; it represents a plan of a portion of the Lower Grindelwald glacier, with both marginal and transverse fissures drawn upon it. I have placed it under the section so that each part of it may show in plan the portion of the glacier which is shown in section immediately above it. It shows how the marginal crevasses remain after the compression of the centre has obliterated the transverse ones; and how the latter join on to the former, so as to form continuous fissures, which sweep across the glacier in vast curves, with their convexities turned upwards. The illusion before referred to is here strengthened; the crevasses turn, so to say, _against_ the direction of motion, instead of forming loops, with their convexities pointing downwards, and thus would impress a person unacquainted with the mechanical data with the idea that the glacier margins moved more quickly than the centre. The figures are intended to convey the idea merely; on the actual slopes of the glacier between twenty and thirty chasms may be counted: also the word "compression" ought to have been limited to the level portions of the sketch.
Besides the two classes of fissures mentioned we often find others, which are neither marginal nor transverse. The terminal portions of many glaciers, for example, are in a state of compression; the snout of the glacier abuts against the ground, and having to bear the thrust of the mass behind it, if it have room to expand laterally, the ice will yield, and _longitudinal crevasses_ will be formed. They are of very common occurrence, but the finest example of the kind is perhaps exhibited by the glacier of the Rhone. After escaping from the steep gorge which holds the cascade, this glacier encounters the bottom of a comparatively wide and level valley; the resistance to its forward motion is augmented, while its ability to expand laterally is increased; it has to bear a longitudinal thrust, and it splits at right angles to the pressure [strain?]. A series of fissures is thus formed, the central ones of which are truly longitudinal; but on each side of the central line the crevasses diverge, and exhibit a fan-like arrangement. This disposition of the fissures is beautifully seen from the summit of the Mayenwand on the Grimsel Pass.