lead to the supposition that this means of transit will ever be very much extended. Where practicable, river navigation appears to be extensively used, and the success or otherwise of the works at present under construction with the view of opening up the navigation of the Godavery, will probably determine whether or not similar operations shall be carried out on other rivers. There is also no doubt that by the introduction of an improved class of steamers many rivers might successfully be navigated, which contain too small a depth of water for the majority of the vessels hitherto in use; three such steamers, calculated to draw only 12 inches of water, are now in course of construction for the river Godavery. Should they prove successful on that river, it is not unlikely that they will be followed by others on the same plan for employment on various streams hitherto not navigated by steam, and on which only vessels of very shallow draught could be employed. With respect to railways, it is not probable that any very great extension of the present system of high-cost lines will be sanctioned, except where they may appear necessary for political or military purposes, and we look rather to a development of the principle of light railways, which may readily be laid down along existing lines of road, and ultimately perhaps, when the traffic has become sufficient to justify the expense in a commercial sense, they might be completed substantially so as to unite with the existing system of high-speed railways, just as it is customary to make a fair-weather road in the first place, and afterwards complete it, by bridging and metalling, as a first-class road. IV. ICE MARKS IN NORTH WALES. By ALFRED R. WALLACE, F.R.G.S., F.Z.S., &c. ONE of the most interesting branches of modern geology, and that on which recent researches have thrown most light, is the inquiry into the exact modes by which the present surface of the earth has been produced. When we see a vertical precipice, a deep chasm, or huge masses of shattered rock, our first impression is to impute these effects to some violent convulsions of nature, such as volcanic eruptions, earthquakes, or floods. It is, however, now generally admitted that such causes have had, for the most part, little if any effect in modifying the surface, except when many times repeated during long periods of time; and it is every day VOL. IV. D becoming more certain that even the grandest and most romantic scenery of mountainous countries has been produced by the slow but long-continued action of those natural causes which we see daily at work, but whose effects during the few years that we can observe them are almost imperceptible. These canses are, the ocean waves, running water, rain and frost; which, if acting for long periods during which subterranean forces are also at work slowly elevating and depressing large tracts of country and to some extent fracturing and loosening the rocky strata, seem capable of producing all the chief features which the surface of the earth now presents in non-volcanic regions. There are, however, a considerable number of very remarkable phenomena which none of these canses will account for, and which appear to have been overlooked or thought unimportant till about twenty-five years ago, when the celebrated naturalist Agassiz visited this country after having carefully studied the effects of modern glaciers in the Alps. He it was who first showed that they could be all explained to the minutest detail by the hypothesis of a recent "glacial period," during the continuance of which the mountains of Wales, Cumberland, and Scotland were covered with perpetual snow, and sent down glaciers into most of the valleys, and sometimes even into the sea. At first this hypothesis was received with incredulity and derision, since it completely contradicted the almost universal belief of scientific men that the earth had been for ages cooling, and that all preceding epochs had been warmer than the present one; but it very soon worked its way even among the most sceptical inquirers, till at the present day there cannot be found a geologist who denies the reality of the "glacial epoch," or the correctness of that interpretation which explains many peculiar features of our own mountain scenery by the agency of ice. A great deal has since been written by geologists and physicists on the effects of ice-work, but comparatively little has been given to the general public; and as the subject is at this time again attracting much attention, owing to new applications of the theory which have given rise to much discussion and are greatly stimulating inquiry, and as it requires little or no previous knowledge of geology to understand either its facts or its theory, I have thought that a popular account of such prominent glacial phenomena as are observable in all our chief mountain districts would be acceptable to many readers of this periodical. We may conveniently consider the chief evidences of a glacial period under the following heads: 1st, The drift; 2nd, Moraines; 3rd, Roches moutonnées'; 4th, Grooved and striated rocks; 5th, Boulders and perched blocks; 6th, Alpine lakes;-and in this order I propose to record the few observations I have made during a month spent near Snowdon and Cader Idris last autumn, incorpo rating briefly what has been observed elsewhere, and adding some account of the more interesting problems and discussions to which they have given rise. 1st. THE GLACIAL OR NORTHERN DRIFT.-This is a layer of loose materials-gravel, clay, mud, pebbles, and angular stoneswhich is found spread at intervals all over Northern Europe, and is very common in the valleys and upland slopes of North Wales. It is very abundant all round the town of Dolgelly, where it forms undulating slopes, mounds, and hummocks in most of the valleys, filling up the space between the flat alluvial meadows on the river side, and the steep rocky slopes of the adjacent mountains. Wherever this is cut through in making roads or railways, it is seen to be full of blocks of stone, pebbles, and large masses of rock, distributed through it without any order or arrangement, the top, middle, and bottom being alike in composition. From the contour of the surrounding mountains it can be often seen that this deposit is of great though very variable thickness, probably often exceeding a hundred feet, and it certainly covers many hundred square miles of country in North Wales alone. On ascending the mountains it is often found on their less precipitous slopes and in the upland valleys, at more than a thousand fect elevation; and it has even been traced around Snowdon by Professor Ramsay to a height of more than two thousand feet. The materials of which the drift is composed are various. Sometimes the rocks are nearly all those of the surrounding mountains, at other times they are such as must have been brought from a great distance. The geological age of the drift is determined by its overlying all, even the most recent formations, and by its containing occasionally marine shells of an arctic type and of species which are all now living. Here we have materials of a loose and miscellaneous nature which were deposited in the sca but not by the sea. That the drift was deposited in the sea is proved by the marine shells which have been found in it up to the height of 1,300 feet on some mountains of Carnarvonshire; and we have thus a proof that North Wales was at a very recent epoch sunk to at least that depth beneath the ocean. The presence of the drift itself, however, at a height of more than 2,000 feet, would prove a much greater submergence. That the deposit could not have been made by the sea, is shown by the want of arrangement of the materials and the abundance of large angular fragments of rock. Water always sorts the materials it deposits. The rocks, the pebbles, the shingle, the sand, the mud, are carried different distances, and deposited in different places or in different layers. Water deposits are stratified. Neither can rocks be carried far by water and retain their angles and clean fractured surfaces. They get rounded into boulders or pebbles, whereas many of the rocks and stones found in the drift are as sharp, angular, and irregular as the blocks and masses which are detached by the winter's frost, and lie under an inland precipice. The solution of this curious problem of the origin of the drift, is to be found in the history of glaciers and icebergs. When a valley is filled with ice, the rocky debris from its slopes and precipices fall upon the surface of the glacier. A quantity of the earth and stones of the bottom of the valley is also forced into the crevices or frozen to the bottom of the icy mass. Now when the ice-filled valley terminates in the sea, large fragments of the glacier break off and become icebergs, and floating away carry with them their load of earth and rocks, which are deposited where they melt, or topple over, or are stranded. In the North Atlantic as far as icebergs float, there must be an annual deposit of matter on its bottom exactly of the same nature as the drift, while in Hudson's Bay and the Gulf of St. Lawrence it must be accumulating still more rapidly. When North Wales was one or two thousand feet lower than at present, it must have formed a group of islands, among which icebergs would frequently become entangled and deposit their loads of foreign matter. At the same time Snowdon and Cader Idris would have been sending down glaciers into the sea, which would spread the debris of their precipices and valley bottoms on what are now the upland slopes and low valleys, but which then were submerged banks and ocean straits. As the land rose above the sea to its present elevation, rivers, floods, and glaciers would more or less furrow and clear away the drift from the valleys, and leave it distributed in the irregular manner in which we now find it. The mere presence, therefore, of this unstratified mass of earth, rocks, and boulders would of itself prove a recent glacial period; since it clearly indicates the existence of icebergs and glaciers in seas and countries where they are now never found. 2nd. MORAINES.-Every modern glacier carries upon its surface more or less of the débris of the rocky valleys through which it passes. As the glacier moves downwards, these are carried with it, and at its termination, where its waste by melting exactly balances its downward progress, this debris must necessarily be deposited, and form a more or less regular heap of rock and earth called the terminal moraine. These moraines are sometimes destroyed almost as fast as they are formed by the streams which issue from the glacier itself or by torrents from the adjacent mountains, but under favourable conditions they remain, and long after the glacier has entirely disappeared tell the tale of its former existence. If owing to a steady change of climate a glacier retires regularly, the moraine-heaps will be distributed over the whole surface its terminal ice-cliff has successively occupied; when on the other hand it is stationary for a considerable time, the debris accumulates to some height, and forms a well-defined terminal moraine. Some of the moraines formed by the old Swiss glaciers, when they stretched far down into the plains, are enormous. That of Ivrea in North Italy is many miles in extent, and 1,500 feet high-a mountain of débris brought down by a glacier which was sixty miles long. There is no other natural agent which can form on level ground such regular mounds as these moraines, many of which resemble artificial earthworks. Their presence becomes therefore a very certain indication of the former existence of glaciers. In North Wales many very perfect moraines may be observed. Around Snowdon in particular they are very abundant, every one of the valleys which radiate from the central peak of the mountain exhibiting them more or less distinctly. These are all described in Professor Ramsay's little work on the Old Glaciers of Switzerland and North Wales; and I can bear witness that, far from exaggerating, he has hardly dwelt sufficiently on the wonderful clearness and well-marked character of this phenomenon. The most striking of all are perhaps those of the Cwm Glass Valley, which descends from the north of Snowdon to the pass of Llanberis. At the mouth of this valley, close above the road which ascends the pass, is what seems to the passer-by a steep rocky hill, but on viewing it from an elevation about a quarter of a mile lower down, it is seen to be a huge longitudinal roof-shaped mound of almost perfect regularity, scattered over with angular blocks of rock; and whose position, with regard to the sides and bottom of the valley, shows it to be an addition—something put there-and having no relation to the proper contours of the surface. Higher up the valley is a small but wonderfully perfect moraine, which stretches across in a regular curve, and of almost uniform size and height, so that when standing on it one can hardly help believing it to be an artificial fortification. But the huge angular blocks of rocks scattered about it, and the other signs of ice-work all around, with the wild loneliness of the situation, and its inferiority as a defensive position to many other points near it, utterly forbid this supposition. The best example of the wide-spreading of rocky debris by the gradual retreat of a glacier, is to be seen in Cwm Brwynog. Under one of the blackest precipices of Snowdon lies the little green lake Llyn dur Arddu, on the other side of which rises a steep ridge, most likely partly rock and partly moraine. Beyond this ridge extends for nearly a mile a gradually-sloping upland, so thickly covered with blocks of rock, often of large size, that from a distance the herbage can rarely be seen between them. In this case every one of these rocks must have been carried across the valley of the lake and deposited where it now lies, and no other natural agent can be found or imagined |