II. Physiographical.* For a detailed account of the physiography of the area adjacent to the River Cass, see Speight, 1916. (A.) General. The lower portion of the Cass Valley alone concerns us here. This area consists of a flood-plain about 7 km. long by 2.5 km. wide. The actual river occupies a comparatively small strip in the centre of the plain, while on either side there extend, east and west, a series of low, flat terraces. Strictly speaking, the term “bed” is applicable only to that part of the valley actually covered by the flowing water. But the Cass, like most New Zealand rivers, is continually liable to flood, at which time the river covers a much wider strip than usual. The river, moreover, even normally, flows as a network of anastomosing streams, and in time of flood these channels become wider and meet, until there is a considerable breadth of running water. In this paper the term “river-bed,” following the colloquial usage, is used to include that part of the valley liable to be covered with water in times of severe floods. This “river-bed” is bounded on either side by a broad expanse of flat land, chiefly tussock-covered, termed colloquially “river-flat,” but in this paper “terrace”. This area is at no time subject to complete inundation, and probably only in times of very severe floods will the lower-lying portions of it be slightly submerged. Now, it is the whole flood-plain which supports the cushion- and mat-plants dealt with, but the chief area, the richest and the most interesting, is the river-bed itself, only a few species of the cushion types growing on the terraces. The river-bed consists of various areas varying from bare shingle to a well-defined plant-association. These areas merge into one another, but certain well-defined stages occur, and for purposes of comparison and reference it has seemed advisable to adopt some scheme of classification of the various portions of the river-bed. The lowest grade may be termed “bed proper”. Above this are various grades which, becoming more and more consolidated and more and more peopled by plants, finally merge into the mature “terrace.” These grades are termed “transition terrace,” and are placed in three grades, called respectively transition terrace 1, 2, and 3. It must be clearly understood that these grades merge into one another. (B.) The Bed Proper, and the Terraces. (Plate I, figs. 1 and 2.) The bed proper is obviously the simplest grade to define; it is not necessarily the lowest part of the valley at any cross-section. It consists of a mass of water-worn greywacke rock the constituents of which vary from fine sand to boulders 0.25 m and more in diameter, the larger ones projecting strikingly above the general surface. It is through such shingle that the river flows, being confined to various channels which are continually changing. An old channel may be deserted for a new one; and the older one, if not used considerably during frequent floods, soon commences to merge into transition terrace. The chief point about the bed proper is its absolute freedom from a plant-covering. The transition terrace, grade 1, is the first stage after the unpeopled bed; it is sparsely covered with plants. It is that part of the river-bed no longer liable to be used as a channel except in case of moderate floods. It has a