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Disregarding for the present the small patch of Glenkiln Shales last described, it is evident that the strata of the Moffat series exhibited in the Main Cliff of Dobb9s Linn are naturally arranged in two main divisions palaeontologically distinct. Each of these divisions, again, falls naturally into two well-defined mineralogical subdivisions, each of which is, in turn, composed of several subordinate bands or zones, individually distinguishable by peculiar lithological and palaeontological features.

In the neighbourhood of St. David9s, the Arenig rocks, described by Mr. Hicks in his paper read at the last meeting of the Geological Society*, are the earliest in which Graptolites are known to occur; and yet, when once they appear, so diversified are their forms that these Pembrokeshire rocks are only equalled, in the number and variety of the genera they contain, by the Canadian Graptolite-bearing rocks of equivalent age known as the Quebec Group. In more ancient deposits two species only, belonging to one of the two great sections into which these fossils are divided, have hitherto been detected, viz. Dictyograptus (Dictyonema) socialis , Salter, and Dendrograptus Hallianus , Prout. The former occurring in the lower portion of the Tremadoc rocks of North Wales, and the latter in the equivalent strata (the Potsdam Sandstone of America), it is impossible to say which genus is the earlier, or whether the group is first represented in Britain or in America. Before the discovery, in 1872, of the extensive series of Graptolites which characterize the Lower Arenig rocks of Ramsey Island, the Skiddaw Slates of Cumberland were supposed to be our earliest Graptolite-bearing rocks; but it is now known that the lowest rocks of the Arenig Group exposed in the vicinity of St. David9s, in which Graptolites abound, are of greater age than any part of the Skiddaw Slates yet described; and it is highly probable that they are also older even than the lowest beds of the Quebec Group known to contain

No sooner were these ancient strata studied in earnest by geologists, instructed by the world-renowned achievements of Murchison in Siluria, than what appeared to be clear and unmistakable proofs of a definite sequence were discovered. The earlier investigators soon recognized the fact that, when viewed upon a large scale, they dipped apparently N. and S. from an axial line running from Berwick to Dumfries. In the neighbourhood of this line they had a very ancient appearance, reminding the geologist of the strata of the Longmynd, which, in Siluria, rise out from below the base of the Silurian system. A further resemblance was found in the fact that they were similarly destitute of all traces of organic life. Precisely as in Siluria, the rocks which surmounted these barren beds in the south of Scotland were found to contain an abundance of the little understood fossils known as graptolites. These graptolite-bearing strata were subsequently ascertained to form the floor of the whole of the Uplands to the north of the unfossiliferous beds, except near the town of Girvan in Ayrshire. There certain limestones and schists were discovered, crowded with fossils, the majority of which were identical with those of the Caradoc formation of Wales, which was known to succeed the great graptolite-bearing Llandeilo formation of that country. To the north of these Girvan beds, higher Silurians were detected in the Pentland Hills and at Lesmahagow, crowded with Upper Silurian fossils, and passing upwards into rocks belonging to the succeeding system of the

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 Palaeozoic 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 palaeontological 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 Palaeozoic 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 Palaeozoic 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 palaeontological 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 Palaeozoic 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 (Ab4), 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 palaeontology 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.

I. Introductory. The Lower Silurian rocks of Scotland, largely developed as they are in the south, and in spite of the great labour that has been bestowed upon them, are by far the least known of all the fossiliferous formations of this country. While the maps of the Government Geological Survey are coloured in all the subdivisions of the strata of the other formations included in the area, the Lower Silurians are merely indicated by a common purple tint, and not the slightest attempt at a subdivision is made. Even the single bed of limestone they contain below the horizon of that of the west coast is doubtfully referred to the Llandeilo, and the sign of interrogation is carefully placed before its title. Nicol, Harkness, J. C. Moore, and many other eminent geologists, have worked different portions of these ancient deposits since the publication of ‶The Silurial System,″ but as yet very little progress has really been made in correlating its different parts with those of the type formation of the sister country. Now this want of accurate information is due to two causes: 1 st, The strata are so rapidly folded and contorted, and are consequently so intricate and confused, while at the same time they present so few differences lithologically, that anything like a physical separation of its masses is well-nigh impossible; and, 2 d, The whole formation, if we except of a few isolated bands of limestone towards the west, and some bands of anthracitic shale that make their

The graptolites obtained by Mr Henderson from the shaly Silurian beds of Habbie’s Howe, and which I have had in my possession for determination, belong at least to three distinct genera. The most frequent fossil (marked 1 on the pieces of shale exhibited) is a peculiar form, which is usually held to be merely a variety of Monograptus colonus (Barrande). It differs from the typical form in several minor respects, such as the shape of the thecae, the direction of the apertural spines, the rate of taper in the polypary, c in the Wenlock rocks of South Roxburghshire, and in the Wenlock and Lower Ludlow of the typical Siluria. The variety under notice has been

In the first edition of his famous work, the “Systema Naturae,” published in 1736, the great Swedish naturalist, Linnaeus, proposed the name Graptolithus for certain natural markings occasionally found upon stones, which are not true fossils, but are mere superficial stainings, having the deceptive appearance of being organic remains. In the twelfth edition of the “Systema,” however, Linnaeus unwittingly figured and described two true fossils under the head of Graptolithus. One of these, his Graptolithus scalaris, was founded upon some strange markings found upon pieces of broken stone collected in Scania, in the south of Sweden. In 1821, Dr. Wahlenberg collected some specimens of the Graptolithus scalaris from Linnaeus’ original localities. These he submitted to a careful examination, and reached the conclusion that they were the remains of fossil Orthoceratites. Through the subsequent researches, however, of Nilsson, Portlock, and Barrande, who discovered and studied many genera and species of fossils of the same type as the original Graptolithus scalaris, it has been demonstrated that these so-called Graptolithus or Graptolites are the petrified remains of Hydroida, allied to the modern Zoophytes of our sea-coasts. These fossil Graptolites are all plant-like forms, and are of many different shapes—linear, leaf-like, tree-like; simple and compound. They all show peculiar notches or teeth upon their outer margins, and are now transformed into shining carbonaceous matter. They are separated by palaeontologists into two main groups—the Rhabdophora and the Cladophora. The Rhabdophora stand almost alone in the animal world in the possession of a peculiar rod or