Page images
PDF
EPUB

merged, and covered by from 30 to 40 feet of sand and scrobicularia clays.

The extent of the previous submergence is unknown.

10. That a re-emergence took place, bringing up the scrobicularia clays and submerged forests once more into dry land the land-surface of the present.

These clays form the bulk of our "fens," "carses," "levels," and other marine alluvia with remains of existing sea-fauna.

Arranging the results—that is, the accumulations and deposits—chronologically, we have:

1. Scrobicularia clays and beach-gravels.

2. Submerged forest-growths.

3. Newer brick-clays.

4. Karnes and ozars in valleys.

5. Moraines or gravel-spits in glens.

6. Older brick-clays.

7. Upper or pebbly till.

8. Lower or bouldery till.

9. Glaciated rock-surfaces. 10. Pre-glacial gravels.

EECENT FOEMATIONS.

RECENT OK FOST-TERTIARY FORMATIONS—LATEST CHAPTER IN GEOLOGICAL HISTORY—FLUVIATILE: DALES AND RIVER-TERRACES— LACUSTRINE: ALLUVIAL FLATS AND STRATHS—ESTUARINE: PLAINS AND DELTAS—MARINE: SILTS, SAND-DRIFTS, AND RAISED BEACHES —CHEMICAL: MINERAL PRECIPITATES AND EXUDATIONS—ORGANIC: VEGETABLE AND ANIMAL GROWTHS—VOLCANIC: LAVAS, TUFAS, ETC.—EXTENT AND VARIETY OF RECENT FORMATIONS— THEIR STUDY THE KEY TO THOSE OF EARLIER DATES—CHRONOLOGICAL ARRANGEMENT OF—HISTORIC AND PRE-HISTORIC—HUMAN AND PRE-HUMAN.

As the glacial epoch, with its bouldery clays and gravels, formed a limit to the tertiary system—over a large portion of the northern hemisphere at least—so within the same latitudes it constitutes an equally decided basement for what are usually termed the Post-tertiary, or recent formations. Of course, like other appellations in geology having reference to time, these terms are merely relative, embracing accumulations that have taken place within the current century, and others that may have been formed fifty or a hundred thousand years ago, but still recent when compared with those of the tertiary and other preceding systems. This is all that is meant by the title at the head of the present Sketch; and whether we adopt Quaternary, Posttertiary, or Recent—designations all employed by geologists in describing these formations—it matters little so long as it is understood that the events referred to have taken place subsequent to the glacial era. These events, recent though they be, present a curious but difficult chapter in worldhistory; curious, as displaying more clearly than the older formations their whole origin and progress, but, like modern human history, difficult of narration, from the exuberance and nearness of the details. Approaching our own times, their interest is proportionally increased, and he who understands them aright cannot fail to catch by reflection a clearer insight into the cycles and systems that went before. In their origin and formation we see a repetition of the origin and formation of all the older formations, hence their instructiveness as a study; while in their superficial dispersion they become the immediate source of sustentation to the plants and animals that inhabit the terrestrial surface.

Arising from the operations of waste and reconstruction described in Sketch No. 2, these Eecent, or, as they are sometimes termed, Superficial Accumulations, will be as multifarious as the agencies concerned in their formation; and hence perhaps the most intelligible way of treating them is to arrange them according to the agents more immediately concerned in their production. In this way we will have Fluviatile formations, or those arising from the action of rivers; Lacustrine, or those formed in lakes; Estuarine, in estuaries; Marine, in seas; Chemical, arising from chemical action; Organic, from the growth and decay of plants and animals; and Volcanic, from the internal fireforces of the globe. There will be older and younger, of course, among these different formations—some so old as to embed the remains of plants and animals no longer inhabiting the same localities, and others so recent as to belong entirely to the current age, and indeed to be still in process of formation. To display them, whatever their age, in intelligible order, is the object of the present Sketch; and he who bears in mind the operations of waste and reconstruction described in a previous paper, can have no difficulty in following the narration of this, the most recent chapter in geological history.

Among the most obvious of Eecent Formations are those produced by the action of rain and rivers. Whatever the winds and rains and frosts loosen and disintegrate, the stream carries onward and downward to lower levels. Were there no great rivers, the debris worn from the mountains would accumulate mainly along their bases, but the runnels gave it to the streams, the streams by their union to the river, and the river carries it forward to be scattered over the plains, to be deposited in lakes, or borne out to the depths of the ocean. Geologically speaking, what is strictly Fluviatile is laid down by the streams and rivers along their courses; and there is not a river in the world that does not present, in some portion or other of its course, patches of meadow-land, holmes, dales, and other flats, that have been formed by the debris carried down by its current. These alluvial flats are generally very heterogeneous in their composition—loamy and clayey silts, sand, gravel, and shingle, with here and there the embedded but often imperfectly preserved remains of terrestrial plants and animals. In course of ages, as the river deepens its channel, and cuts its way from side to side down the valley, the older of these flats will stand higher and higher above the current; and thus it is that along most rivers there are sets of terraces marking the heights at which they formerly ran, and the levels over which they spread their inundating waters. It is usual to arrange these terraces into Mgher river-gravels and lower river-gravels—the former of vast antiquity, and rarely containing organic remains, and the lower of more recent origin, and containing the remains of plants and animals, some of which have long since become extinct in the regions where their relics now occur. It is from the lower and middle of these terraces in Britain, France, and other European countries, that the bones of the mammoth and woolly rhinoceros have been exhumed, along with the flint implements of rude and primitive races; and were the river-deposits of the other continents geologically examined, there is no doubt they would exhibit in a similar way a gradation from the events of the current century back to those bordering on the tertiary epoch. As it is, these river-deposits play an important part in the physical geography of every country, their rich and well-watered surfaces presenting the finest fields, whether for forest-growth, pasture,or cultivation. We have only to name the principal rivers of the world to recall to the geographical reader the alluvial expanses that mark the most of their courses, and these in magnitude according to the volumes of the respective rivers and the flatness of the country through which they flow. And even where magnitude is not concerned, these river-deposits are not without their importance. Every gully from the mountain-side has its terminal spit of sand and gravel, and it is often in such deposits, worn from the cliffs and veins above, that we find the most abundant and readiest supplies of the gems and precious metals—as witness the diamond-conglomerates of Brazil and India, the tin-gravels of Cornwall, and the gold-sands and shingle of California, Columbia, Australia, New Zealand, and the Oural. These auriferous sands and gravels are but the debris of the older mountain-cliffs disintegrated by the frosts and rains, and carried down by the streams to the lower "creeks" and "gullies," where, accumulating for ages, they are often of great thickness, and carry us, in many instances, far back even into the tertiary period. Many of the Australian goldgravels, indeed, are surmounted by thick overflows of basaltic lava; and as volcanic agency has long ago ceased to manifest itself in these regions, these show at once the vast

« PreviousContinue »