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On the Silurian Rocks of the English Lake District

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On the Silurian Rocks of the English Lake District H. HICKS ON CAMBRIAN AND SILURIAN CLASSIFICATION. 105 The Arenig Group, which comes in next in the Table, is divided into an Upper and a Lower series, both characterised by a distinct assemblage of fossils, The Lower series occurs chiefly in the neighbourhood of St. David's, and is of about 1,000 feet in thick­ ness. The fossils consist of numerous graptolites, especially dendroid forms, and trilobites of the genera Ogygia, Trinucleue, and .k:glina. All the forms are thoroughly Silurian in character, and the series marks a definite line for the base of the Silurian groups. The Upper series comprise the well known Skiddaw Slates. Upon the Arenig comes the Llandeilo Group, the lowest rocks recognised by Sir R. Murchison in the typical Llandeilo district. This group in the Table is divided also into an Upper and Lower series, marked out by distinct faunas. The Bala or Caradoc Group now comes in with its exceedingly rich fauna, and it forms in the Table the upper member of our Lower Silurian, and also of Professor Sedgwick's Upper Cambrian. Sir R. Murchison added to the Lower Silurian the Lower Llandovery rocks, but Sir Charles Lyell in his" Student's Manual of Geology," 1871, preferred "to regard them as the base of the Upper Silurian, to which group they are linked by more than twice as many species as to the Lower Silurian." The evidence of Professor Hall and others, in America, tends to prove, however, that ultimately it will be necessary to make a Middle Silurian division, and though the group is scarcely important enough, as hitherto known in this country, to warrant such a division, yet should a spot be found where the present unconformity is filled up, a series may be discovered, like that in the American continent, sufficiently large to enable it to be done. The higher groups, Wenlock and Ludlow, which complete the Table, are each capable of being divided into at least two groups, with a considerable thickness of strata and well defined faunas. 2. ON THE SILURIAN ROCKS OF THE ENGLISH LAKE DISTRICT. By H. ALLEYNE NICHOLSON, Esq., M.D., D.Sc., M.A., F.R.S.E., Professor of Natural History and Botany in University College, Toronto. Considering the extent to which the Silurian series of the Eng­ lish Lake District has been studied, it is somewhat remarkable that there should still be considerable difference of opinion as to the 106 PROF. H. A. NICHOLSON ON THE LAKE DISTRICT. ages of its various members, their correlation with the deposits of other areas, and, in some cases, even their actual succession. In the following communication I propose briefly discussing this sub­ ject, and in so doing I shall speak of the following groups of strata :- 1. The Skiddaw Slates. 2. The Borrowdale Series, or Green Slates and Porphyries. 3. The Ooniston Limestone and Associated Shales. 4. The Graptolitic Mudstones. 5. The Ooniston Flags. 6. The Coniston Grits. 7. The Kendal Rocks. I. THE SKIDDAW SLATES. The lowest sedimentary rocks of the Lake District are unques­ tionably those to which Professor Sedgwick applied the name of " Skiddaw Slates." This group comprises a great series of dark cleaved shales, with intercalated flaggy beds, which reach a total thickness of probably not less than about 7,000 feet. The upper beds of the series are generally readily distinguished from the lower, by being darker in colour, by being of a more fissile or shaly nature, and by being usually very imperfectly, or not at all, cleaved. As yet, however, no accurate line has been drawn between the upper and lower portions of the series, and some of the most characteristic fossils are common to both. As to the relations of the Skiddaw Slates to the rocks which succeed them (the Borrowdale series), ithas been generally believed that the highest beds of the Skiddaw series were conformably over­ laid by the base of the Green Slates and Porphyries, and this opinion is still held by high authorities. I have endeavoured, however, to show (Geol. Mag., vol. vi., March, 1869) that this is not the case, and that the Skiddaw Slates are unconformably sur­ mounted by the basement-beds of the Green Slate series. To this opinion I still adhere; but having no fresh evidence to bring for­ ward in support of my views, I need not press this point at present. As regards the palreontological relations of the Skiddaw Slates, it has long been known that they contain a very rich and varied graptolitic fauna, comprising many peculiar forms. No less than nine genera and thirty-two species of graptolites have been de- PROF. H. A. NICHOLSON ON THE LAKE DISTRICT. 107 tected in the Skiddaw Slates; and five genera (viz., Dichograpsus, Tetraqrapsus, Loqanoqrapsus, Phyllograpsus, and Triqonoqrapeus ), are exclusively confined to this horizon. All these genera, how­ ever, and a large number of the species, occur in the so-called "Quebec Group" of Canada; and there can, therefore, be no reasonable doubt as to the contemporaneity of the SkidJaw and Quebec groups. The most important fossils yielded by the Skid­ claw Slates, besides graptolites, are, Lingula brevis, the peculiar phyllopod Caryocaris Wriqhtii, and the trilobites, lEglina bino­ dosa, ./Eglina caliginosa, Agnostus Morei, Phacops Nicholeoni, and Trinucleus Gibbsii. In applying the stratigraphical and paleeontological elements at our disposal to the solution of the question of the age of the Skid­ daw Slates, we find ourselves beset with unusual difficulties. The physical evidence only tells us with certainty that the Skiddaw Slates underlie strata which at the utmost cannot be older than the Llandeilo Flags of Wales. If the Skiddaw Slates were conform­ able to these, there would be the probability that the Skiddaw series is not older than the Arenig rocks of Wales, as is believed by Professor Harkness. Even in this case, however, the Skiddaw Slates can hardly be said to be the equivalent of the Arenig group, as the latter contains none of the peculiar graptolites of the for­ mer. If, on the other hand, the Borrowdale series (as I believe), rests unconformably upon the Skiddaw series, there is a high pro­ bability that the latter is older than the Arenig, and that it cor­ responds to part of the Upper Oamlrians. The paleeontological evidence, though more satisfactory than the physical, still leaves much to be desired. The great majority of the Skiddaw grapto­ lites occur in no other British formation, aud do not, therefore, supply any element of comparison so far as Britain is concerned. No stress, also, Can be laid upon the occurrence of some four or five of the Skiddaw graptolites in the Arenig and Llandeilo de­ posits of Wales. The phyllopod Caryocaris Wrightii, so abundant throughout the whole Skiddaw series, is also peculiar to this hori­ zon, and affords, therefore, no means of comparison. The tri­ lobites, on the contrary, belong to the Silurian genera, or at least to genera all of which are represented in the Silurian series. The genus Phacops, in particular, has not yet been detected in rocks undoubtedly older than the Silurian. There are no "primordial" trilobites j and the evidence, therefore, to be drawn from these 108 PROF. H. A. NICHOLSON ON THE LAKE DISTRICT. crustaceans would lead us to believe that the Skiddaw Slates are referable to the Lower Silurian series. On one point only do the fossils of the Skiddaw series speak with certainty, and that is as to the equivalency of the British formation with the Quebec Group of Canada. All the pecnliar genera of graptolites in the Skiddaw Slates occur also in the Quebec Group, and in Canada, as in Britain, they do not occur in any younger formation, with rare exceptions. At least half of the Skiddaw species are also identical with Canadian forms.· We may, therefore, safely conclude that the Skiddaw and Quebec groups are " contemporaneous" deposits, in the geological sense of this term. It follows from this that the age of the Skiddaw Slates might be fixed without difficulty, if only the age of the Quebec Group were known with certainty. Unfortunately, however, the precise age of the Quebec rocks is still a matter of opinion, for they contain a mixture of ,. primordial ,. fossils with genuine Silurian types, and their stratigraphical position is also somewhat uncertain. As regards the first of these points, we meet in the Quebec Group with the Silurian genera Asaphus, Cheirurus, Illamus, and Lichas, mingled with" primordial" trilobites of the genera Dikelocephalus, M enocephalus, Bathpurue, and Bathsiurellue. The brachiopods, gasteropods, and cephalopods are, on the other hand, referable to types which are characteristic of the Lower Silurians. The Quebec Group has generally been regarded as belonging to the same formation as the Calciferous Sand-rock of the State of New York, which immediately overlies the Potsdam Sandstone, and the age of which appears to' be indubitably Upper Cambrian. In Newfoundland, however, the Quebec Group (" Levis formation") has been described as being separated from the" Calciferous" byover two thousand feet of other strata. If there be no disturbance of the strata this would show that the Quebec Group is considerably younger than the Calciferous; but it is not impossible that the phenomena admit of some other explanation. II. BORROWDALE SERIES. Overlying the Skiddaw Slates comes the great series of rocks termed by Professor Sedgwick the" Green Slates and Porphyries," for which the name of " Borrowdale Rocks" has been proposed by Professor Harkness and myself. These consist essentially of a vast succession of bedded traps and porphyries, with stratified felspathic PROF. H. A. NICHOLSON ON THE LAKE DISTRICT.
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