The Girvan Succession. Part I. Stratigraphy
Authors: Lapworth Charles 
Published: October 2009

Summary of the Foregoing Evidences and Conclusions Respecting the Stratigraphy of the Girvan Succession

(1)

In the foregoing pages I have laid before the reader all the more important data obtainable in the geographical area under examination which bear upon the main question of the natural order of the Lower Palæozoic rocks of the Girvan region. The original arrangement of the beds themselves has been so frequently interrupted by profound dislocations, and has been rendered so dubious locally by perplexing folds and inversions, that the task of reducing them to their natural order has been one of far more than ordinary difficulty. But so well are the several subformations in the collective series individualized by distinct petrological features, that the field-geologist has generally little hesitation in recognizing their entangled or dislocated fragments at a glance. At the same time also the foldings and inversions of the strata prevail only in certain definite geographical subareas, where they can usually be ultimately detected and allowed for. Finally, the greater faults are, as a rule, by no means difficult of localization, owing to the fact that they bring into abrupt and unnatural collocation upon the ground strata very distinct in their lithological and palæontological features.

Nevertheless the complexities and difficulties of the stratigraphy of the fossiliferous rocks of the Girvan region are so great that the solution of the problem of their natural sequence has been only arrived at by the accumulation of an excessive amount of evidence collected in the field. This evidence, however, is now so full, and so conclusive, that there can be no longer any doubt of the natural petrological subdivisions of the strata of the Girvan rocks, or of their true positions in the ascending succession.

(2)

The physical evidence, as developed in the preceding pages, in support of our conclusions may be summarized as follows;—

(i)

Selecting the remarkable Benan-Hill Conglomerate as our primary horizon of reference, we discover that it is merely the central member of a series (the
Barr Series
) of boulder-beds and conglomerates, with intercalary zones of limestone and fossiliferous shales. This series exhibits proofs of the most perfect conformity from base to summit, and its various members admit of minute and complete investigation in the field. This Barr Series includes the well-known Craighead or Stinchar Limestone as a subordinate member near its base, where it is divided from the underlying Ballantrae rocks by a calcareous conglomerate and breccia of irregular thickness.

(ii)

The tumultuous Barr Series is everywhere conformably surmounted, in the sections of the Girvan region, by a second series (the
Ardmillan Series
), composed throughout of Graptolitic flagstones and shales, which nevertheless arrange themselves naturally in several distinct subformations of well-marked petrological characters. The higher and lower divisions of this great series (the Ardwell, Barren Flagstone, and Drummuck Beds) have their respective systematic positions fixed by incontestible stratigraphical evidences. The proper relations of its central divisions (the Cascade and Whitehouse beds), the strata of which are usually inverted, are established mainly by geographical considerations.

(iii)

The Graptolitic series of Ardmillan visibly underlies a third series (the
Newlands Series
), consisting of
Brachiopod
-sandstones,
Pentamerus
-grits, and
Monograptus
-shales. The natural place of the first (Mulloch-Hill beds) division of this series is fixed beyond dispute by its relation to the terminal beds of the older Ardmillan series, and that of the highest division (the
Camregan
group) by its relation to the newer
Dailly Series
. The systematic position of the central division (the Saugh-Hill group) is deduced with equal certainty from its intermediary place in the series; but the sequence of the component strata of this central division is rendered so dubious by inversion, faulting, and local unconformities, that we are unable to give more than a provisional classification of its minor zones.

(iv)

Finally, we discern a fourth petrological series (the
Dailly Series
), at once the thickest and most homogeneous series in the Girvan succession. Its place at the summit of the whole is established by the circumstances that it forms a single series of similar strata, which is wholly distinct from either of the series below, while it lies on the southern (or upper) side of the Newlands Series, from the highest zones of which its strata appear to graduate in conformable sequence.

(3)

In place of an enigmatical group of Lower Palæozoic rocks of no great vertical thickness, varying locally in their petrological characters to an extraordinary extent, and containing an admixture of fossils elsewhere characteristic of formations of several distinct geological epochs, as believed by some of the earlier students of these beds, we find an orderly arranged sequence of strata several thousands of feet in vertical thickness, grouped very naturally in successive formations of distinct petrological features, each formation retaining even in its subordinate zones the same characters over the entire area, and, as we shall show in the second part of this memoir, invariably affording the same special group of fossils.

(4)

In brief, our study of the stratigraphical relations of the rocks of the Girvan succession has fully established the following propositions:—

i.

The Girvan succession of Lower Palæozoic rocks consists of a generally continuous series of more or less fossiliferous strata of a collective thickness of 7000 feet.

ii.

It is divisible into four main rock-formations, each of which is individualized by special petrological and palæontological characteristics.

iii.

Each of these formations is, again, made up of several subordinate members, whose relations to the subformations above and below are beyond dispute, and which retain their special characteristics both in rocks and fossils wherever they are laid open for investigation within the district.

The detailed classification of these Lower Palæozoic strata of Girvan, as developed in the preceding pages, is given in the Table, fig. 31, pp. 660, 661.

(5)

These Girvan rocks appear to repose, at their base, upon the generally older igneous and altered rocks of Ballantrae. The Ballantrae rocks have, as yet, been too imperfectly studied to allow us to hazard any conclusion respecting their true geological age. That many of the rocks grouped together under this title are of far greater antiquity than the basement-beds of the Girvan succession may be regarded as established by the fact that fragments of the Ballantrae rocks occur in the Kirkland or Purple Conglomerate at the base of the Girvan sequence. These pre-Girvan traps and ashes must either represent the Arenig and Llandeilo volcanic rocks of Wales and Cumberland or must be of more ancient date. On the other hand, rocks which are unquestionably of true Girvan age occur at many localities within the typical Ballantrae region itself, while the patches of altered or so-called Ballantrae rocks found outside that area, as at Shalloch Hill, Laggan Hill, and elsewhere, almost certainly include some greatly altered Girvan rocks.

(6)

The sequence among the Girvan fossiliferous rocks is broken by at least one fairly distinct unconformability, viz. that at the, base of the Craigskelly conglomerate; but the presence of boulder-beds at the base of the Mulloch-Hill group, at the base of the Saugh-Hill Grits, and elsewhere, renders it exceedingly probable that other local stratigraphical breaks may eventually be detected.

These local unconformities, however, can be of no great systematic importance; for the general gradation, both in sediments and fossils, from the base to the summit of the Girvan succession is practically complete. Each distinct petrological formation in the vertical series is connected with its neighbours, both above and below, by a group of beds intermediate both in physical and in zoological features. Thus the very distinct formations of the Stinchar Limestone and the Benan Conglomerate graduate into each other through the transitional zone of the
Didymograptus
-beds (Ab
4
), the Benan and Ardwell series through the transitional Balclatchie group, the Ardwell and Whitehouse beds through the intermediary Cascade beds, and so on. Even the two grand divisions of the succession, the Upper and Lower divisions of the Girvan rocks, are united by the intermediary formation of the Mulloch-Hill beds.

The geographical distribution of the various members of the Girvan succession within the region we have described is given in the accompanying Maps and Plates, &c.

The detailed description of the physical structure of the region, and of the lithology and palæontology of the several members of the Girvan succession, together with the discussion of their resemblances, physical and zoological, to their extra-Girvan equivalents, are points deferred to the second part of this memoir.