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hemisphere (in Patagonia, Australia, and New Zealand) has ever undergone a similar glaciation; and if so, whether it took place, as in the northern hemisphere, immediately preceding the current era.* These and similar questions must receive satisfactory answers before a generally approved theory can be hoped for; and geology, in this as in other instances, will best attain her end by a diligent accumulation of facts, and the widening of her field of observation.
Such is a hasty, but, we trust, not unintelligible Sketch of the glacial or ice epoch—that strange period in recent geology when the frost-giant that now reigns supreme within the polar circles laid his iron grasp on the northern hemisphere down to the 40th or 42d parallel of latitude. How or by what means this crisis was brought about—whether by some peculiar distribution of sea and land, or by some great secular recurrence in the earth's planetary relations —theorists are not agreed; but there is no difference of opinion as to its existence, and none as to the long continuance of its sway. Its presence is visible on every hillside and in every glen in the British Islands, Northern Europe, and North America; in the rounded eminences, in the polished and striated and grooved rock-surfaces, in the moraine-like mounds of gravel that bar the glens, in the huge rounded and striated blocks that lie scattered over the ground, and in the thick tenacious bouldery clay that envelops so much of the lower and level tracts of the country. No known agent, save ice, could have produced these ap
* From recent reports by the provincial surveyors of New Zealand, as well as from Mr Darwin's well-known descriptions of South America, it would appear that the southern hemisphere has been subjected to a similar phase of ice-action. The further investigation of this as to contemporaneity with that of the northern hemisphere, would materially assist in the framing of an acceptable hypothesis.
pearahcesi—ice on land, and ice on -water; ice, in fact, such, as we now behold on the higher mountains of the globe, and within the arctic and antarctic regions.
We have arranged the period into three stages—the first, when the prergjacial land (somewhat higher than the existing continents) began to receive the ice-sheet; the second, during which the ice-bound land subsided to the extent of 1800 or 2000 feet; and the third, during which the land was step by step re-elevated, and the ice gradually disappeared.* Each of these three stages must have left its own proper impress; but the second has obliterated so much of the first, and the third so much of both, to say nothing of what has been subsequently effaced by frosts and rains and rivers, that it is always extremely difficult, and often impossible, to discriminate their results. Hence the great difficulty of reading aright the phenomena of the glacial epoch; and hence the conflicting views entertained by geologists respecting their origin and arrangement. This much, however, is certain, that the pre-glacial or pliocene land-surfaces, wherever they are found, contain fossils; that the first stage of the ice-epocb is characterised by boulders little removed from their parent rocks, by finely glacialised rock-surfaces, and by the true boulder-clay or "till" of Scottish geologists, and is always unfossiliferous; that the second stage is characterised by reassorted clays,
* While, for the sake of distinctness, we thus divide the ice-epoch into three great stages, it must be borne in mind that there may have been minor and local oscillations of sea and land during each successive stage. Since the close of the glacial period such oscillations have taken place more than once in our own islands, as proved by the "submarine forests " that occur at so many places along the existing coasts—these forests, now partially under the sea-level, having evidently grown at a higher elevation, been submerged to receive the silts that now cover them, and again upraised to their present levels. Such minor oscillations tend to complicate, but they do not obliterate, the broader phenomena of a period.
by more rounded and widely-dispersed boulders, and is also unfossiliferous; while the third stage has more moraines, ridges of sand and gravel, terraces with occasional shells, and finally, in the lower levels, the silty clay or "brickclay," containing boreal shells, star-fishes, bones of seal, whale, northern ducks, and other kindred remains. The local differences may not be always ascertainable; the general order above sketched is unmistakable throughout the British Islands.
Cold and dreary, and inimical to life, as the ice-epoch must have been, it has left its impress on every foot of the surface to which its limits extended. The rounded outlines of oar hills, the gentler mouldings of our glens, the scooping-out of many of our higher lake-basins, the undulating gravelly surfaces of our broader valleys, the terraciform southern and south-eastern slopes of our mountains—ninetenths, in fact, of that which gives character and colour to our northern scenery—are the direct results of its long-continued sway. Much has no doubt been since obliterated by the frosts, rains, and running waters of the current era, but the broader features chiselled out by the ice-epoch still remain, reminding the spectator at every turn of its presence, and the long continuance of its power.
EESUME OF THE GLACIAL EPOCH.
The following chronological arrangement of the stages of the Tertiary and Ice epochs may further assist the conception of the general reader, while it cannot fail to be of use to the student of geology:—
1. That the climate of our latitudes was sub-tropical during the Eocene period.
The palm-fruits, palm-stems, crocodilian and chelonian remains found in the London and Paris basins.
2. That from the Eocene to the-close of the Pliocene— that is, throughout the Tertiary period—there was a gradual decline of climate.
The laurel, sycamore, poplar, chestnut, and other trees of the miocene beds; and the oaks, firs, alders, hazels, and willows, &c, of the pliocene.
3. That at the close of the Tertiary period our latitudes began to be enveloped in perennial ice; and that the land then stood considerably higher than at present.
The absence of flora and fauna in most of the pre-glacial accumulations, and where they do occur, their boreal character (hairy mammoth and rhinoceros); and also the pre-glacial rivercourses (of Newcastle, Ayrshire, and Grangemouth) being several hundred feet (300 or more) beneath the present sealevel.
4. That while the land stood at this higher level, the glacial epoch culminated or was at its greatest intensity.
This the period of the true "boulder-clay" or "till"—the products of land-ice = "ice-mantle."'
5. That immediately after the culmination of the ice
•period the land began to sink, and continued slowly to do so
till it was about 2000 feet below its present level.
During this submergence much of the boulder-clay (the terrestrial boulder-clay) was re-formed as it came in contact with the sea during the land descent; and that then was formed the "upper or pebbly till," which is partly the result of floated ice.
6. Tliat when the land had reached its greatest submergence the climate seems to have become milder, and also that then an upward movement of re-emergence took place.
The lowlands being now freed from the ice-sheet, and as the land gradually rose, much of the "boulder-clay" and the "pebblyclay" was acted on by the free-flowing sea, and carried out by tides and currents and deposited in deeper water, forming the "brick-clays," which are all less or more fossiliferous—the fossils being chiefly those of moderately deep waters.
7. That as the climate gradually improved, and the land continued to rise, the ice retreated to the highest mountain glens—this being the period wJien our "straths" and "curses" were shallow estuaries of the sea, receiving in their deeper portions " clays," and in their shallower "sands and gravels."
It was at this time when most of the "kames" and "ozars" were formed in the lowlands, and when the "moraines," terminal and lateral, were left in the higher mountain-glens.
8. That as the gradually-rising land attained a height about 150 feet higher than at present, the glaciers disappeared from the higfiest hills, and a climate somewhat warmer than at present was introduced.
It was at this time that the "submerged forests" grew on all the fertile portions of the seaboard, and that a flora and fauna of a "warm-temperate" climate prevailed over the whole of the northern latitudes of Europe and Asia.
9. That after a long enjoyment of this warmer climate the land again began to sink, the forest-growths were sub