![Thumbnail: [Volume 52, 1920 page 97]](/journals/rsnz/images/rsnz_52/thumbnails/rsnz_52_00_0147_0097_ac_02.gif)
Relation to Geological Problems.
We may now consider what conditions are indicated as most probable in the remote past of this community in general and of R. paucifolius in particular.
It seems inconceivable that the plant should have “originated,” established itself, and subsequently maintained itself for countless ages, all within the narrow limits of its present distribution, and the first condition requisite for its establishment would be the existence of a very much larger area of continuous Tertiary limestone strata than is now to be found anywhere in New Zealand.
This area need not have been—and, indeed, could not have been—one continuous sheet of limestone beds covering the whole of the district within which the isolated fragmentary remnants now exist. But the inference here drawn from the existence of this whole calciphile unit, and of Ranunculus paucifolius in particular, is that these beds must once have been more extensive and more nearly continuous than they are now. The ancestral Ranunculus may well have existed upon soils of pre-Tertiary origin and developed there its xerophytic characters, while one form of it established itself especially upon the limestone, developed characters accordingly, and ultimately become virtually incapable of maintaining itself elsewhere. This is, at any rate, one hypothesis which seems to fit the facts. But the exact sequence of events can here, in the nature of things, be only a matter of conjecture.
![Thumbnail: [Volume 52, 1920 page 98]](/journals/rsnz/images/rsnz_52/thumbnails/rsnz_52_00_0148_0098_ac_02.gif)
This comparatively wide area must have had, at some remote period, a steppe or semi-desert climate, under whose influences a xerophytic and partly calciphile flora developed and flourished, and it is likely that what we now have represents only a portion of this flora, many species having probably died out altogether.
This area must have been partly a peneplain (upon which alone R. paucifolius, it would seem, could “originate” and flourish), and would probably be conterminous with a range or ranges of hills with limestone rocks exposed and weathering into dust exactly as they now do on the small area here under observation. But such peneplain need not have consisted entirely of Tertiary limestone beds.
The area would be in the nature of a strip or belt, of no very great width and probably much interrupted, corresponding roughly to the shoreline or lines of the hypothetic Tertiary sea or seas. It would be conterminous with and more or less alternated with an area or areas of pre-Tertiary formation, probably lying to the north and east, as posited, e.g., by Cockayne (1911, pp. 343–44), by way of which probably the mesophytic flora would return when a more humid climate should prevail in this area. Upon this pre-Tertiary area the related species, R. chordorhizos, &c., would have originated and flourished, or that single species or form from which they and R. paucifolius trace their common descent.
The greater part of these limestone beds was destroyed by erosion of various kinds in subsequent ages, leaving only the present small isolated remnants, of which the Trelissick Basin is one of the largest.
It is impossible that by the elevation of the land 3,000 ft. or 4,000 ft. (Haast, Hutton, Park), and the consequent refrigeration and glaciation, the whole flora of the district (as has been thought) was driven to another tract, now non-existent, and returned with the subsidence of the land and consequent change of climate. “Return” of a calciphile flora over areas upon which the Tertiary beds had becn destroyed would be impossible, especially since, as we have seen, this flora as a unit is not a “traveller”; and we cannot escape the conclusion that this plant community has been represented within the area of the small basin, since it first established itself or “originated” in that neighbourhood.
Glaciation bears upon the question in two ways:—
(1.) Hutton (1900, p. 176), followed by Cockayne, correlated the supposed drought epoch, of which our flora shows signs, with the glacial epoch, which he placed in the older Pliocene period. This view was adopted by Cockayne (1901, pp. 280 et seq.); but that authority believed that at the height of the glaciation the eastern mountains (within which this area is included) might still support a xerophyte flora like that of the shingle-slips of the present day (Cockayne, 1911, pp. 348 et seq.).
The view of Speight (1911) and others is that the last glacial epoch is much more recent, that the drought period was correlated with it (Cockayne, 1911, p. 344), that the Tertiary deposits were continuous over a much larger area than is the case now (Speight, 1915, p. 54), that the Castle Hill area probably escaped glaciation altogether (Speight, 1917, pp. 323 et seq.), and that the Trelissick Basin at the height of glaciation was “probably a snowfield” (Speight, 1917, p. 323).
It would seem certain that a steppe climate or period of drought must have obtained here over a large area at least once (probably more than once) since Tertiary times, but to the present writer it seems quite uncertain whether this was coeval with and resultant from the glacial epoch or not
![Thumbnail: [Volume 52, 1920 page 99]](/journals/rsnz/images/rsnz_52/thumbnails/rsnz_52_00_0149_0099_ac_02.gif)
and the analogy of other lands would seem to show that such a climate may have existed in New Zealand independently of any glacial epoch, whether that epoch be (with Hutton) older Pliocene or (with Speight and others) Pleistocene. The question whether the “drought” be Pliocene or Pleistocene is here dwelt upon because, whatever conditions obtained and whatever balance was established at the end of the “drought,” if that “drought” were Pliocene that balance must in all probability have been disturbed and a new set of condition reached when the later Pleistocene glacial period came. The problem is then, to discover what were the conditions during and after some more recent period, rather than during and after the exceedingly remote period of any possible Pliocene glaciation and concomitant steppe climate.
(2.) Glaciation also has been supposed to have been the chief, though not the sole, eroding agency by which the great area of Tertiary beds was destroyed (Hutton, 1885, p. 92; Speight, 1915, p. 337). The question of the agency by which, and the probable period during which, these beds have been destroyed is, however, one of secondary import in this connection. It is enough, for the botanical problem, if it is decided that they once existed, have been in one way or another largely destroyed (being now represented by the small isolated fragmentary areas which remain to us), and that the Trelissick Basin (including the small area here studied) escaped glaciation and any great degree of refrigeration during any glacial epoch. We may then imagine the ancient birthplace and habitat of Ranunculus paucifolius and its associates to have been a semi-desert area of flat or flattish plains diversified with ridges and islets of higher ground, and neighboured closely by a range of limestone hills or even mountains. The whole landscape would have a yellow hue; upon the surface large areas of unstable shifting debris would possibly alternate with ridges of more grassy and closed formation. Strong winds would be frequent and dust-storms violent. The vegetation would be sparse and harsh, including the species here described, and no doubt many others which have perished; a pale-purple, greyish, and brown colour scheme would predominate. The land would be occupied by no animals save lizards and birds, its whole appearance being monotonous, parched, and glaring; while the dreariness of the scene would be enhanced by the setting of pallid limestone rocks of grotesque and fantastic form—chessmen, collar-studs, sea-lions, and gorilla torsos. The general appearance of the limestone desert might be much like parts of the Sahara—e.g., as figured in plate 345 of Schimper's Plant-geography, p. 614.
If Speight's (1911) hypothesis of a pluvial climate in post-glacial times be accepted—and certainly the-evidence collected by him seems to be conclusive—this community and others like it must have passed through and survived such a period, unless the districts in which they exist have been specially favoured. There is little or no reason to suppose that this was so, for, although Cockayne (1900) mentions that the Trelissick Basin is now very dry climatically, old residents do not support this view; and, in any case, the fact, if established, that it is now dry does not prove that it was always so in the remote past.