Humboldt were to man a four-oar up the St. Lawrence, through this valley into the valleys which were the American lakes, how the whole crew would pride themselves on their cross fractures,' and and rectangular intersections of geodesic lines!'

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As Lake Superior would have been the main recipient of alluvium, they might perhaps find little to indicate that the lower valleys had ever been lakes. And how on earth,' says Mr. Hopkins, 'is Colonel Greenwood going to make this placid stream cut a channel through yonder limestone mountain ridge thirty miles in length ?' Here Lyell, looking serious, says, To tell you the truth, gentlemen, 50,000 years ago I foretold in the seventh edition of my Principles," page 202, that the river would do this.' Well,' says Mr. Hopkins, its raather late to contradict this now, but you must, because in your "Elements" you stood by me and my mathematical earthquakes in the Weald. And in the very "Principles" which you quote, you have ventured on mathematical earthquakes of your own, in Italy. Page 246 you have formed the course of the Anio on my igneomathematical plan.'

66

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The Anio flows through a deep irregular fissure or gorge in the Apennine limestone

which may have been caused by earthquakes. In this deep narrow channel there existed many small lakes, three of which have been destroyed since the time of history, by the erosive action of the torrent, the last of them having remained down to the sixth century of our era.'

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be of igneous formation.

dicts himself

ject.

And indeed it is hard to see, if the 'fissure or Lyell contragorge' of the Niagara is of aqueous formation, on this subwhy the fissure or gorge' of the Anio should For in both cases there is the same cause, a river, working on the same material, limestone, producing the same effect, levelling the dams of lakes.

'I shall not deny what I have printed,' said Lyell, nor can you; and you will allow, that, however often I have taken one side of this question, I have equally often taken the other. For instance, page 255; "If we ascend, for example, the valley through which the Dranse flows, we find that it consists of a succession of basins, one above the other; in each there is a wide expanse of flat alluvial lands, separated from the next basin by a rocky gorge, once evidently the barrier of a lake. The river has filled those lakes one after the other, and has partially cut through the barriers, which it is still gradually eroding to a greater depth. The examination of almost all valleys in mountainous

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districts affords similar proof of the obliteration of a series of lakes by the filling up of hollows, and the cutting through of rocky barriers, a process by which running water ever labours to produce a more uniform declivity." There!' said Lyell, I am right either way; for my sword cuts both ways.' In Mr. Hopkins's quick reply, the words were heard, 'better a sword should cut only the right way.' But Mr. Martin and Humboldt interposing, adjourned the debate to that day 50,000 years.

I have said that the denudation of the Weald and that of the valley of the St. Lawrence each illustrates one of two opposite ways in which nature works the same effect, namely, the driving rivers through ridges which cross their courses. It must be confessed, however, that to level and perfect the waterslope of the line of American lakes is a more difficult job than to make that of the Weald. For in the Weald the water has always worked from above, on soft materials, and with any number of streams, while the St. Lawrence has had, single-handed, an up-hill work, through the very hardest materials. It has had to fill the lakes before it could erode the lowest part of their brims.

But though water cannot flow up hill, it can by ponding flow over hills, and so cut channels

through hills, and this, as Lyell says, is a very universal process in nature. This universal process is, however, directly opposed to the submarine theory.

lakes are

being laid dry

by rain.

Truth, if not always, is at least often para- The American doxical, and paradoxical as it may appear, it is nevertheless true, that rain and rivers are laying dry these, the largest bodies of fresh water in the world. And that by a double process also paradoxical. First, by filling them with alluvium. Second, by emptying them by eroding

their barriers.

With regard to the first cause, the filling them in, Lyell says, 'the streams which discharge their waters into Lake Superior are several hundred in number, without reckoning those of smaller size. On the northern side, which is encircled by primary mountains, the rivers sweep in many large boulders with smaller gravel and sand, chiefly composed of granite and trap rocks.' The more central deposits consist of 'finer mud' and adhesive clay.' With regard to the second cause, the emptying of the lakes by the erosion of their barriers, that this is going on, is proved by parallel terraces, marking their former levels far above their present levels. A depth of fifty feet has been thus taken from the surface of Lake Superior, which is nearly 2,000 miles in

Two miniature

Weald valleys

Wight.

waves, and showing lines of lithodomous perforations-facts which indicate most clearly the successive upheaving of the land, since the sea was inhabited by the existing species of testacea.'

Suppose the land to have risen 100 feet above the level of the sea. This would have given a constant rapid fall from the river's mouth to the sea, and the river would thus gain the power of lowering its bed. While this was going on, no delta could be formed. And where rivers have estuaries instead of deltas, it may possibly in some cases, be owing as much to the rise of land as to the wash of marine currents. In the case of the St. Lawrence, for instance, if a delta had been in formation before the land rose, as it rose the river must have cut a channel through the delta to the depth of 100 feet.

There are two miniature Weald valleys in the in the Isle of Isle of Wight, which, like every other spot on earth, witness to the transition of the earth into the sea. The north chalk, from the Needles to Culver Cliffs, is equivalent to the North Downs. The south chalk, St. Catherine's Hill, is equivalent to the South Downs. Doubtless, this south chalk formerly ran east and west, parallel to the north chalk; but it has been washed away by the sea. The loss of the two ends of this southern barrier allows the escape of the waterflow to the