The only known locality for Ranunculus paucifolius is a rock-bound hollow behind the farm buildings at Castle Hill, in the Trelissick Basin, about a mile and a half from the homestead of the late J.D. Enys, upon whose property the farm was situated.
A full account of the general geological features of the district is given by Speight (1917), with a map showing the Castle Hill itself (p. 323), and plates, of which Plate xxi, fig. 1, gives a view of the small hollow from above.
The locality of the species is a small synclinal basin forming a kind of amphitheatre. Its main direction is north-east and south-west, the north-east end being the higher. It is bounded on the south and west by the steep grassy slopes of Castle Hill, with frequent outcrops of limestone (seen in Plate IV), and on the north and east by piles of limestone rocks from 80 ft. to 100 ft. high, which are weathered into the usual fantastic shapes. It is entered from the eastern side by a gap in the limestone barrier about 100 yards broad; a small but constant stream rises on the south-west side of the basin, and flows through this gap on to the flat cultivated plains of the Castle Hill farm, which are overlooked by the steep limestone rocks. Except at this point the basin is surrounded on all sides by limestone rocks or steep slopes of grass upon a limestone soil. The weathering of the rocks by frost and wind produces a great amount of debris, which is blown far and wide by the strong winds of the Southern Alps, and this debris collects in the basin owing to its enclosed character. Within the basin a small dune-system is produced by the action of the wind, so that its floor is diversified by small
ridges and shallow hollows of dune type. The south-west half of the basin is clothed with tussock grassland, and does not concern us. The north-east half, at the south-west end, shows first (moving from south-west to north-east) a small area, about 120 yards by 100 yards, of open debris formation which does not harbour this Ranunculus. The upper (or north-eastern) portion consists of a larger area of limestone debris, about 350 yards by 100 to 150 yards, of which some parts are clothed with a half-closed tussock formation, others with an open formation, including the Ranunculus paucifolius, while some considerable portions are entirely barren. The bottom of this part of the basin is occupied chiefly by a belt of half-closed tussock formation; the eastern side has rapid slopes of coarse debris below the limestone rocks; the western side (shown in Plate IV) has a gentler gradient, and the grass-covered slopes of Castle Hill here ease off gradually into the central basin. Tongues of half-closed tussock formation, on this side, occupying higher ground or ridges, separate roughly circular or semicircular areas of the open formation well seen in Plate V, within which most of the plants of Ranunculus paucifolius occur.
The debris itself is of a flaky character, but is reduced, over most of the area, to a fine uniform powder. The colour of the bare patches is thus a pale yellow, deepening to brown in certain places, owing apparently to the volcanic element present in the limestone itself in varying quantity. The debris on the steep eastern slopes is much of it very coarse and rough, and very large flakes of the stone lie thickly here.
At the extreme north-east corner a dune formation is being broken up. Here are semicircular breaches of the higher dune, whence masses of very loose debris come down. At the top the slope is steep and the material deep and soft; hardly and vegetation can grow, and the line separating the tussock grassland from the perfectly barren space is sharp and clear.
Possibly all parts of the basin have at one time or another been thus closely covered, the covering being subsequently stripped away or buried, while a certain area must always have remained sufficiently open somewhere in the area for the calciphile community to exist.
Digging at a spot where several plants of Ranunculus paucifolius grew close together showed that the limestone debris was here exactly 18 in. deep. At that depth a more consolidated subsoil was reached. Down to this depth the material was perfectly uniform, fine and incoherent, and the roots of the Ranunculus, about 10 in. or 12 in. long, do not reach beyond this layer, which seemed fairly damp throughout at the end of a period of about a fortnight's fine weather. In a really dry season this material must, of course, become extremely dry.