82 



KNOWLEDGE. 



March, 1915. 



peralure the magnesium is expelled, and a residue containing 

 most of the borou in a crystalline state is left. 



CRUDE NITROGEN IN NATURAL GASES.— The 

 crude nitrogen fractions separated from natural gases, such 

 as fire-damp and the gases emitted by thermal springs, 

 have been examined by MM. Moureu and Lepape {Comptes 

 Rendus, 1914, Volume CLVIII, page 839). The analyses 

 show that the crude nitrogen is remarkably constant in its 

 composition, whatever its origin, and is composed of very 

 similar proportions of nitrogen, argon, xenon, krj'pton, and 

 heUum. From this fact the conclusion is drawn that these 

 constituents have a common origin, possibly, in the nebulous 

 period. Variations in their quantities may be attributed 

 to diffusion or other physical processes. 



NICOTINE FROM WASTE TOBACCO LEAVES.— 

 A description is given by MM. Chuard and Mallet (Schweiz. 

 Apoth. Zeit., 1914, Volume LII, page 424) of the method 

 used in Switzerland for the manufacture of nicotine from 

 the leaves rejected as unsuitable for the preparation of 

 tobacco. The usual process is to extract the nicotine from 

 these leaves immediately after the removal of the crop of 

 choice leaves, but the experiments cited show that this is 

 a mistake. If the stripped stems be left in the ground, and 

 the soil treated with sodium nitrate, there will be a further 

 growi;h of leaves, and consequently an increased yield of 

 nicotine. For example in some cases the amount of the 

 alkaloid obtained was increased by as much as seventy- 

 seven per cent. 



WTien the nicotine was extracted from the plants at 

 once the average yield was 0-725 gramme per plant, whereas, 

 when the leaves were allowed to grow again, the yield was 

 increased to 1 -284 gramme. 



It was proved that the sodium nitrate was not directly 

 responsible for the production of nicotine, but that it acted 

 indirectly as a fertiliser, stimulating the growth of the plant. 

 The proportion of nicotine showed considerable variations 

 in different parts of the same plant, much more being present 

 in the roots and shoots than in the stems. 



GEOGRAPHY. 



By A. Scott, M.A., B.Sc. 



STREAM-VALLEYS AND THEIR MEANING.— The 

 three chief processes operative in the development of the 

 " thalweg " of a stream are vertical down-cutting, lateral 

 cutting, and " sweep " or the down- valley migration of 

 the meanders. The first of these is dominant when the bed 

 of the stream is considerably higher than the local equi- 

 librium level, so that the stream tends to cut vertically 

 downwards through the subjacent rocks. The result is 

 typically a narrow gorge, with steep sides, and following 

 the course which the stream had before the initiation of 

 the dovm-cutting. Wlien the gradient is low, the second 

 process is the one which is most active. The stream tends 

 to swing from side to side in its valley, so that a differential 

 wear on the channel ensues. As is well known, the maximum 

 deposition occurs on the inside of the meanders, and a 

 complementary corrosion on the outside. This seems to 

 be mainly due to the current tending to move in a tangential 

 direction at the bends, this tendency increasing with large 

 volume and low gradient. The ultimate form, therefore, 

 is a scalloped outline, wth large circular meanders sym- 

 metrically arranged. The comparative rarity of this form 

 is due to the operation of the third process, which, in general, 

 is most important, and leaves the greatest ultimate impress 

 on the shape of the valley. In addition to the tangential 

 flow at a bend, there is a strong tendency, due to the down- 

 stream component of gravity, for the stream to take the 

 shortest course, ?.£•., that round the inside of the bend. The 

 actual course of the strongest current is the resultant of 

 these tivo, and this explains not only the asjonmetric erosion 

 of the meanders, but also the tendency 'of the latter to 

 migrate down-stream. Circumstances which favour this 



migration are, low gradient with but little down-cutting, 

 and a large volume of water carrying coarse material. 

 In a river with intrenched meanders the solid rocks resist 

 all three processes, but in a broad valley with rock sides 

 the latter oppose lateral cutting, while the soft alluvial 

 material of the valley-flat offers little resistance to 

 sweep. 



The rate of upUft is often an influential factor in 

 determining which of these processes has had the greatest 

 effect. If the uplift be rapid, down-cutting becomes pre- 

 dominant, and the stream entrenches itself in its original 

 course. If the rate of uplift be equal to, or less than, the 

 original rate of down-cutting, lateral cutting and down- 

 valley sweep come strongly into play, with the formation 

 of broad, regular curves, the outsides of which are usually 

 steep and undercut, and the insides smooth and shelving. 



J. L. Rich (Journal of Geology, July-August, 1914) has 

 classified valleys into three types — Open ^'alleys, Intrenched 

 Meander Valleys, and Ingrown Meander ^'alleys — and has 

 considered the formation of each type in terms of these 

 processes. The Open Valley may either be straight or 

 meandering, with wide, open curves and steep sides. The 

 Intrenched Meander Valley, which is apparently the same 

 as the Incised Meander type of other authors, is " one 

 whose stream, having inherited a meandering course from 

 previous erosion cycles, has sunk itself into the rock with 

 little modification of its original course." The windings of 

 the stream follow those of the valley, and river-flats are 

 conspicuously absent. The Ingrown Meander Valley is 

 one which has either developed a meandering course or 

 has expanded an inherited one. This type is characterised 

 by steep, undercut sides on the outside of the curves, and 

 gentle deposition slopes on the inside. 



The first of these types is formed when a comparatively 

 straight stream undergoes a rapid uplift. The stream 

 entrenches itself by down-cutting, which continues till 

 grade is reached, when lateral cutting comes into play. 

 This, however, is soon superseded by a down-stream 

 migration of the bends, and the final form is either a broad, 

 open valley, with flat bottom and long, flat curves, or a 

 steep-walled, narrow V-shaped valley. The Intrenched 

 Meander Valley results from the rapid uplift of a meandering 

 stream, which continues to hold its original course, and to 

 entrench itself deeper into the subjacent rocks as long as 

 down-cutting is the principal factor. This tj'pe is rarely 

 found, because, when the upUft ceases or becomes slow. 

 Ingrown Meander Valleys develop owing to the operation 

 of the other processes. This third tj'pe generally results 

 from the gradual uplift of both straight and meandering 

 streams. The meanders tend to increase through lateral 

 cutting, with the consequent corrosion on the concave 

 sides and deposition on the convex, while sweep leads to 

 asymmetry of the bends, and finally to their down-stream 

 migration. This is particularly the case when uplift ceases 

 and a flood-plain forms. 



Obviously the whole three types may be present in a 

 single drainage system, which, for example, undergoes a 

 rapid uphft. The main stream would form an Open Valley 

 where originally straight, and a valley of the second type 

 where originally meandering, while the tributaries of the 

 upper parts would develop Ingrown IMeander Valleys, 

 as their rate of uplift would be relatively slow. 



GEOLOGY. 



By G. W. Tyrrell, A.R.C.Sc, F.G.S. 



RAISED BEACH FEATURES IN ARR.\N.— The 

 two figures give illustrations of sea-wear during the period 

 of the ten-foot raised beach which fringes the island of 

 Arran. In Figure 69, on page 80, a rock-arch is 

 shown, which has been eroded in the soft Triassic 

 sandstones and grits at Largybeg Point. The view 

 was taken from the sea. The arch occurs just above 

 high-water mark at the extreme seaward edge of the 

 raised beach. It was initiated by the erosion of a 

 soft band, the weathering of which had caused the deep 



