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another against its sloping surface, the lower portion of the newer series may be margined by conglomerates, while the higher portion is not; the latter may consist of shales and sandstones, with perhaps barely a foot of pebbly material at their base. Instances of such overlap with an absence of conglomerates are common in the case of the Carboniferous system; where the Lower Carboniferous rocks lie unconformably on older strata, conglomerates and pebbly sandstones are always present, but where the Coal-Measures overlap the lower group and lie on older beds, they often rest directly on the surface of the latter without the intervention of any conglomerate, and with only a thin pebbly basement bed. So also in the east of England, where an ancient land surface is buried beneath the Cretaceous rocks, the Gault clay overlaps the sandstones and rests on the older rocks with a mere basement bed, which is generally only a foot in thickness.

When such cases occur we may perhaps infer that the extent and height of the land had been so reduced by continued submergence that the area remaining was small and of low elevation above the sea, so that the conditions were unfavourable for the formation of conglomeratic beds. In many cases the tract of land had doubtless been reduced to the condition of an island, an isthmus, or a low promontory.

The Difficulties of Geographical Restoration. These arise chiefly from two causes-the imperfection of our knowledge and the imperfection of the geological record. The first is being gradually removed by the industry of geologists, but there are still many parts of the British Islands about the geological structure of which we really know very little, and there are many others about which more detailed information is much to be desired. Again, there are very large areas where the older rocks exist, but where they are buried and concealed from view by the newer for

mations; thus we hardly know anything of the subterranean geology of the large areas which are covered by strata of Carboniferous age both in England and Ireland. Other areas again are concealed by the Neozoic strata, and we are only just beginning to obtain some knowledge of the underground limits of the older Neozoic rocks, by means of the deep borings which have been made from time to time in the eastern part of England.

The imperfection of the geological record is another great source of difficulty, and one which will never be altogether overcome. The rocks which remain to us as the records of any one period are but a remnant of the deposits which were formed during that period, and yet before we can attempt to restore the geography of that time we must replace in imagination the rocks which are lost, so as to form a conception of the space over which they originally extended. For some portions of the space this may be easily done, as in the case of conformable beds lying on each side of an anticlinal, but when the final limit of a formation toward a given direction is a fault or an abrupt boundary, without even an outlier beyond, we are left in complete uncertainty as to the original limits of those beds in that direction. Under such circumstances we are obliged to fall back on general considerations, such as the lithological changes seen in the exposed areas and the physical characters of the country beyond the present boundary of the formation; still it must be admitted that in many such cases any attempt at geographical restoration partakes more or less of the nature of guess-work.

I must, therefore, ask my readers to remember that some of the restorations attempted in the following chapters, especially those of the earlier periods of geological history, are built on slight foundations, and may have to be modified by the results of new discoveries. In some instances the facts which are known suggest different inferences to

different minds, and there are several cases in which different views are held with regard to a certain area having been above or below water during a certain period. In such cases I have carefully examined the different views which have been taken by those who have written on the subject, before selecting that which appeared to be the most probable interpretation of the facts.

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CHAPTER I.

ARCHEAN TIME.

ELOW the oldest rocks which contain definite organic remains there are in certain parts of the British Islands still older rocks, which are now generally known by the name of Archæan, as being the most ancient rockmasses that have yet been recognized in the earth's crust. They are also called Pre-Cambrian, for their infra-position to the Cambrian rocks is the real proof of their antiquity.

In England, the principal Archæan tracts occur in South Wales (Pembrokeshire), North Wales (Carnarvon and Anglesey), Shropshire (Wrekin and Caradoc range), Charnwood Forest, the Malvern Hills, and the Lizard peninsula. In Scotland, the Hebrides, as well as parts of Ross and Sutherland, consist of Archæan rocks. In Ireland, they occur in Donegal and Galway.

The Archæan rocks are all more or less metamorphic; some are distinctly crystalline and foliated, others are ast clearly volcanic ashes and lavas, while some seem to be of sedimentary origin, such as the quartzites and limestones. The general opinion is that these Pre-Cambrian rocks are divisible into two great groups,' which may or may not belong to distinct systems. The older group consists chiefly of gneiss, which is often so granitoid as to resemble a true granite; the newer group is composed either of vol

1 Callaway, "Geol. Mag.," Dec. 3, vol. ii. p. 258.

canic rocks, felspathic lavas and ashes, or of hypocrystalline rocks, including schists of many kinds, quartzites, dolomites, and altered grits.

Some geologists think that the upper group lies unconformably on the lower, and that they should be regarded as widely separated systems; others think that the apparent unconformity is only a local one connected with the volcanic origin of the newer group; while some—and particularly Dr. A. Geikie-regard the granitoidite as truly igneous rock of later date than the hypocrystalline strata.

It is obvious that while such diverse opinions exist with regard to the relations of the Archæan rocks, little can be said concerning their probable origin and mode of formation, or concerning the physical conditions of the earth's surface at this early period of its history. Mr. J. E. Marr has even ventured to doubt whether any of the Archæan rocks were originally marine sediments; he points to the great amount of volcanic activity which is testified by the intrusive and eruptive rocks of the Welsh, Shropshire, and Charnwood districts, and he suggests that even some of the hornblendic and chloritic schistose rocks may only be metamorphosed tuffs. The limestones are thin, lenticular beds, and might, he thinks, have been formed in lakes, or by the calcareous springs which are common in volcanic districts.

From these and other facts he concludes that Archæan time was one of continued and universal vulcanicity, and that the amount of erosion and denudation which took place at its close was so violent, rapid, and extensive that great masses of rock, which had been metamorphosed at a considerable depth below the original Archæan land, were in many places brought to the surface; while the greater part of the unaltered surface rocks were swept away and re-deposited as Cambrian sediment.

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