in the Philosophical Magazine.' What I particularly wish to call attention to is the fact, that the only theory put forward even by the most eminent of his opponents is, that the depressions in which lakes lie (when they are bounded by rocky strata and not merely dammed up by moraines) have been formed by unequal disturbances of the crust of the earth or upheavals of valley bottoms, and that the ice during the glacial period may have filled up and slightly modified these basins, and also have prevented them from being silted up, but did not form them. In no one case that I am aware of has it been shown that the strata are thus tilted in opposite directions so as to produce a lake basin, nor is any hint given why these tiltings and depressions should have occurred in the proportion of a thousand times in glaciated districts to once in countries that have not been ice-ground.

The suggestion that lakes, however numerous, formed beyond the limits of the ancient glaciers, may have been all silted up and converted into alluvial plains while those filled by ice have alone been preserved, seems at first sight to meet the case, but a little consideration shows that it is quite inadequate to solve the problem. First, we have no right to start with any other assumption than that lakes before the commencement of the glacial period were distributed with some average regularity over the different regions of the globe, if causes so universal as tiltings and depressions of strata were the chief causes that produced them. Secondly, if the present disproportion in the distribution of lakes was caused by those not preserved by ice being silted up, it would show that the process of filling up lakes is almost always very rapid, and therefore that no lakes can be very old. The ten thousands of existing lakes must therefore all have been originally formed just before the commencement of the glacial epoch, and in a time not so long as has since elapsed; and yet, during the whole time that has since elapsed, the process of lake forming must have entirely ceased over more than one half of the globe! Another, though a minor difficulty, is that it is necessary on this hypothesis to suppose that the time the glacial epoch lasted was many times longer than the time which has elapsed since the ice left the lake basins, for we see that the existing lakes have been only to a very small extent silted up, whereas the supposition is that ninety-nine hundredths of the lakes of all the rest of the world were silted up during that period. I have gone a little into this general argument of distribution, because it is one that a man who knows very little either of geology or glaciers may put forward without presumption, and also because it seems to me to have been very much lost sight of in the discussion of this question. We can all see that a true account of the origin of lakes must explain their present most remarkable distribution,

although very few of us may be able to form any sound judgment as to what angle will stop a glacier's motion up hill.

It would appear, then, that there is at all events a strong case in favour of glaciers having had something to do with the formation of lakes. I therefore examined with much interest into the peculiar arrangement and position of the small lakes of North Wales, to see if they gave any support to Professor Ramsay's theory or seemed inconsistent with it. We may conveniently group most of these lakes into:-1st, such as lie in more or less regular bowl-shaped hollows of the mountains; and 2nd, those situated in longitudinal valleys. Immediately beneath the peak of Snowdon are three great chasms, which contain small lakes at an average elevation of 2,000 feet above the sea. On the east is Glas llyn, on the north are the two small lakes of Cwm glas, and on the west are the three little lakes of Cwm Clogwyn. All these lie in irregularly bowl-shaped valleys with a comparatively narrow opening; they all spread out and are larger within than the entrance to them would lead one to expect. Another feature they have in common is a comparative flatness of bottom. From below you have to climb a steep ascent or even a precipice to reach them, but when you have surmounted this you find a rugged undulating surface spreading out to the foot of the precipices which every where surround it. Cader Idris has two somewhat similar chasms containing lakes, and on carefully examining the Ordnance maps we see that there are numbers of such lakes around the higher mountains, occupying lofty bowl-shaped chasms with a more or less narrow exit. One of the largest of this class of lakes is Llyn Llydaw, which is more than a mile long and lies right across between two spurs of Snowdon, which close round it so as to leave a very narrow entrance. How these valleys were originally formed it is not very easy to understand, unless they can be connected with varying texture and resistance of the rocks. The symmetry of their arrangement around or on each side of lofty mountains is against this supposition, and I have been often inclined to think that they must owe their peculiar form to marine action during the various submergences the country has undergone. However this may be, it is evident that such a form of ground being already in existence when the glacial period came the ice must have accumulated in these crater-like hollows to a great height, and pressing forcibly on a nearly flat or undulated bottom while in slow but continued motion outwards, could hardly fail to deepen the basin here and there and thus form the little lakes we now see.

on,

The second class of lakes or those in longitudinal valleys are generally situated at a much lower level, and are as a rule larger than the mountain tarns just described. The two lakes of Llanberis,

together more than three miles long, are good examples of this class, and illustrate very clearly their characteristic peculiarities. There is a drainage into these lakes of about twenty square miles of country, bounded on both sides by mountain ranges over 3,000 feet high. The whole of the glaciers from these had to pass out between the ridge of the Clegr and that which descends from Cefn du, forming a pass about half-a-mile wide, while the shores of the lakes are all along bounded by steep and lofty slopes which would throw the whole weight of the accumulated ice into the nearly level trough between them. That the grinding power here was very great is evidenced by the fact of the shores of these lakes presenting finer cases of striation and grooving, of mamellation, and of complete planing off of the softer rocks, than are perhaps to be found any where else in Wales. Now most of the other lakes show exactly the same arrangement,-wide upland valleys with many tributaries above them, and below them a sudden narrowing of the valley by projecting spurs. This can in most cases be sufficiently seen on the Ordnance maps, but it is still more striking to look down at the lakes themselves from a moderate elevation. Look at the two ridges that meet together at an angle and shut in the valley at the lower end of Llyn Ogwen, or the precipitous slopes that confine Llyn Cwellyn, west of Snowdon, and Talyllyn, south of Cader Idris. In these and most other cases the valleys containing lakes are of very moderate inclination or nearly flat, so that the motion of the glacier would be slow and would chiefly arise from pressure. When therefore a sudden narrowing of the channel occurred, the ice would necessarily accumulate just above the obstruction, and thus give that increased weight and grinding power which are exactly the conditions said to have produced lake basins. Without going any

further into particulars, I may state generally that the situation and surroundings of many of the lakes of North Wales are just such as ought to exist if Professor Ramsay's theory be the true one.

As a glacier can only be now grinding out a lake basin in the very thickest part of its course, it is very difficult to see the operation going on. At the same time so much is known about glaciers, and so many of the facts bearing upon this question are admitted by all, that some conclusions seem quite clear. For example, all admit that glaciers do (or once did) grind down the rocks over which they pass, to some extent. The grinding is caused chiefly by the weight of the glacier, and therefore where the glacier is thickest the grinding will be the greatest. Glaciers behave like a very thick semi-fluid mass, flowing and filling up channels of varying widths, and therefore accumulating where there are obstructions to their free passage. Now where such an accumulation takes place in a valley of tolerably uniform slope, there will be more weight

and more grinding power than elsewhere, and thus hollows must be formed. And a hollow once formed the ice is there so much thicker and the pressure so much greater, and thus the hollow may increase more rapidly the deeper it goes. Then there comes the objection, that when the hollow is deep the ice at its bottom will be motionless, the upper layers sliding over the lower ones.

But who really knows this? It is a pure supposition; and there seem to be as good arguments on one side as on the other. And who, of all our philosophers previous to direct observation, would have supposed that glaciers could flow at all, and retain their form and continuity? The fact seems to be, that these huge ancient glaciers, spreading over hundreds of miles of flat country half-a-mile thick, are too vast for us to say what they could have or could not have done.

It is proverbially hard to prove a negative, and at present there is really no positive theory before the world, except Professor Ramsay's, that in any way explains either the overwhelming proportion of lakes situated in glaciated regions-or the fact that so many of the great lakes of Switzerland and Italy are situated exactly where they should be if they were ground out by glaciers,or that the size and depth of the lakes correspond to the admitted size and thickness of the ancient glaciers. Many who oppose this theory will perhaps say that they admit it to be good as regards the smaller lakes and tarns, but uphold the elevation and subsidence theory for the larger ones. But this will in no way avoid the difficulties of distribution I have already pointed out, since the large lakes are very numerous and, as well as the small ones, abruptly cease before reaching the limits of the ancient glaciers,--limits, it must be remembered, traced before this theory was enunciated, and by men who even now do not all adopt it. Again the lakes form such a continuous series in position, form, and magnitude, that the presumption is against their having been formed by two quite distinct processes. Lakes have, no doubt, been sometimes formed by disturbance, tilting, or subsidence; but these are evidently exceptional causes, and are not to be assumed in any particular case unless they can be proved.

In connection with this subject, I may allude to one main point of difference which has existed among geologists almost since the subject first attracted attention, and which still exists. It is the question whether the glacial phenomena, so abundant over the whole of the northern half of North America, have been produced by enormous aërial glacial masses, covering at once or at different times the whole country,-or by icebergs floating down over it and grating along a shallow sea-bottom. Agassiz first propounded the "glacier" theory, and still upholds it. Sir W. Logan supports the same theory, and Professor Ramsay of course considers

VOL. IV.

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that the vast American lakes are to some extent a proof of it. the other hand Sir Roderick Murchison, Sir Charles Lyell, and Mr. Dawson, all geologists of the greatest eminence, maintain the "iceberg" theory. Exactly the same difference of opinion occurs as to many other countries, such as North Russia, Finland, and even Scotland, but we will now consider America only, because I wish to state one difficulty which I cannot find alluded to in all that has been written on the subject in this country. The iceberg theory supposes that all the lake regions of North America were about a thousand feet under the sea at a very recent period, that the country was then ground and striated by icebergs, and has since risen to its present level. Now the great lakes, Michigan and Huron, are a thousand feet deep, their bottoms being about four hundred feet below the sea-level. When the land rose up these vast basins must have been full of salt water. What has become of it? No doubt it would soon run off at the surface, and be replaced by fresh, but as a mere physical problem, would all the salt water from a thousand feet deep be carried off by the influx and efflux of fresh water? Has water ever been brought up from the bottom of these lakes, and is it as fresh as that of the surface?*

But even if no trace is or ought to be found of the salt-water lakes that must so recently have existed, a difficulty of a totally different nature arises. These lakes and all the lakes and rivers north of them to the Arctic ocean now contain great abundance and variety of fresh-water fishes, and among them are many found in the lakes only and some entirely confined to single lakes. There are about twenty-two of these American lake-fishes described by ichthyologists, most of them quite distinct and well-marked species, found nowhere else in the world. About twelve are confined to the group of the Great lakes, and there is one distinct genus of the perch family (established by Cuvier) which has never been found except in Lake Huron. Now the glacial epoch is postpliocene; that is, it is within the period of existing species. The mollusca were all identical with those now living; the vertebrates have been changed a little, but chiefly by the extinction of some species. How then are we to explain the occurrence of so many peculiar species and one peculiar genus in fresh-water, lakes the whole district around which was so recently under the sea? It may be said that the same difficulty affects the glacier theory, for if that be true, the lakes were only made by the ice and were not in existence till it left them. To this it may be answered that the country round the lakes in every direction was in existence though the lakes were not, and we need not suppose the whole land to have been covered with ice at once. It probably took different directions

* I am informed by an eminent physicist, that by the process of diffusion the whole of the salt water would no doubt in time be carried off.