We

By others they have been thought to have been formed in the boiling ocean, which first condensed upon the still hot surface of the globe. In truth, we are still profoundly ignorant as to the conditions under which they arose. have hardly any means of ascertaining in what order they were formed. We know no method of determining whether those of one region belong to the same period as those of another. Nor can we always be sure that what have been called Archæan rocks may not belong to a much later part of Geological Record, their peculiar crystalline structure having been superinduced upon them by some of those subterranean movements described in chapter xiii.

Of Archæan rocks the most abundant is gneiss, passing on the one hand into granite, and on the other into micaceous and argillaceous schists, with interstratified bands of various hornblendic, pyroxenic, and garnetiferous rocks, limestone, dolomite, serpentine, quartzite, graphite, hæmatite, magnetite, etc. These various materials are more or less distinctly bedded. But the beds are for the most part inconstant, swelling out into thick zones, and then rapidly diminishing and dying out. This bedding somewhat resembles that of sedimentary rocks, and the manner in which the limestone and graphite occur, recalls the way in which limestone and coal are found in the fossiliferous formations. The inference has accordingly been drawn that the Archæan crystalline bands were really deposited as chemical precipitates or mechanical sediments on the floor of the primeval ocean, and have since been more or less crystallised and disturbed. But from what has been brought forward in chapter xiii., regarding the totally new structures which have been developed in rocks by subterranean movement, it is evident that a bedded arrangement and a crystalline texture, like those of the Archæan gneisses and schists, have sometimes been induced in rocks by excessive crumpling, fracture, and shearing. How far, therefore, the apparent bedding of Archæan rocks is their original condition, or is the result of subsequent disturbance, is a question that cannot yet be answered.

The alternations of gneiss and other crystalline masses form bands which are usually placed on end or at high angles, and are often intensely crumpled and puckered, having evidently undergone enormous crushing (Fig. 114). Attempts

FIG. 114. Fragment of crumpled Schist.

have been made to subdivide them into groups or series, according to their apparent order of succession and lithological characters. But such subdivisions, even where practicable, are probably only of local value. As a rule, those members of the system which, if the succession of beds may be trusted, are the lowest and oldest, present coarser crystalline characters than those which seem to be higher and later. They often consist of massive granitic gneiss, with abundant veins and bands of the coarsely crystalline variety of granite, known as pegmatite. The apparently higher rocks are less coarsely crystalline gneiss, and often mica-schists and other schistose masses. No unquestionable relic of organic existence has been met with among Archæan rocks. Some of the Archæan limestones of Canada have yielded a peculiar mixture of serpentine and calcite, with a structure which is regarded by some able naturalists as that of a reef-building foraminifer. It occurs in masses, and is supposed by these writers to have grown in large, thick sheets or reefs over the seabottom. By other observers, however, this supposed organism (to which the name of Eozoon has been given) is regarded as merely a mineral segregation, and various undoubted mineral structures are pointed to in illustration and confirmation of this view. The rocks in which Eozoon occurs have been so greatly mineralised by the processes of metamorphism, that any original organic structure in them could hardly be expected to have escaped destruction. Though the structure in Eozoon is in some respects peculiar, it nevertheless so much resembles some recognised mineral arrangements, that its claim to be regarded as an organism has not been satisfactorily established.

Archæan rocks cover a large area in Europe. In the British Islands, they are principally developed among the Hebrides and along the north-west coasts of the Scottish Highlands, where they give rise to a singular type of scenery. Over much of that region they form hummocky bosses of naked rock, with tarns and peat-bogs lying in the hollows, seldom rising into mountains, but forming the platform which supports a singular group of red sandstone mountains. Here and there, they mount up into solitary hills or groups of hills. The highest point they reach on the mainland is at Stack, near Loch Laxford, which is 2364 feet above the sea. But in the Island of Harris they sweep upwards into rugged mountainous ground, of which the highest summits rise more than 2600 feet out of the Atlantic, and are visible far and wide as a notable landmark.

On the continent of Europe, Archæan rocks have their greatest extension in Scandinavia, where they evidently belong to the same ancient land as that of which the Hebrides and Scottish Highlands are fragments. They range through Finland far into Russia, appearing in the centre of the chain of the Ural Mountains. They form likewise the nucleus of the Carpathians and the Alps, and appear in detached areas in Bavaria, Bohemia, France, and the Pyrenees. They are estimated to occupy an area of more than 2,000,000 of square miles in the more northerly part of North America, stretching from the Arctic regions southwards to the great lakes. In this vast region they have been subdivided into an older series, termed Laurentian, and a younger series, called Huronian. It thus appears that both in the Old and New World, the Archæan rocks are chiefly exposed in the northern tracts of the continents. The areas which they there overspread were probably land at a very early geological period, and it was mainly from the waste of this land that the original materials were derived, out of which the enormous masses of stratified rocks were formed.

In the southern hemisphere, also, ancient gneisses and other schists, referred to the Archæan system, rise from under the oldest fossiliferous formations. In Australia and in New Zealand they cover large tracts of country, and appear in the heart of the mountain ranges. It thus appears that all over the world the oldest known rocks are gneisses and similar or allied crystalline masses, having a remarkable uniformity of character.

CHAPTER XVII.

THE PALÆOZOIC PERIODS-SILURIAN.

THE portion of geological history which treats of those ages in which the earliest known types of plants and animals lived is termed Palæozoic. Of the first appearance of organic life upon our planet we know nothing. Whether plants or animals came first, and in what forms they came, are questions to which as yet no satisfactory answer can be given. The oldest discovered fossils are assuredly not vestiges of the first living things that peopled the globe. There is every reason, indeed, to hope that as researches in all parts of the world are pushed into older and yet older rocks, still more ancient organisms may be discovered. But it is in the highest degree improbable that any trace of the earliest beginnings of life will ever be found. The first plants and the first animals were probably of a lowly kind, with no hard parts capable of preservation in the fossil state. Moreover, the sedimentary rocks which may have chronicled the first advent of organised existence are hardly likely to have escaped the varied revolutions to which all parts of the crust of the earth have been exposed, but have probably been buried out of sight, or have been so crushed and broken and metamorphosed that their ori