28 



KNOWLEDGE 



[Febbuary 1, 1898. 



along tbeir flanks. The fine range of the Malverns — the 

 backbone of the English Midlands — may thus owe much of 

 its pre-eminence to the subsidence of the country to the 

 east, whereby the Trias now forms a lowland which is easily 

 flooded by the Severn ; while the Carboniferous rocks, 

 which cause such mountainous country further north, are 

 safely hidden away far below the reach of denudation. 



The floor of a continent is, then, a reality — something 

 that supports this wrinkled film of scarps and furrows, of 

 level plains and axial ridges, on which we spend our lives. 

 If we cross a continent and an ocean, we say that we have 

 seen something of the world — much as a fly who should 

 contemplate St. Peter's from the weathered surface of the 

 dome. The true world lies beneath us ; and as yet the 

 only certain clue that we possess as to its constitution is 

 its well-determined mean specific gravity. This figure is 

 5-6, as against 2-6 or 27 for the mean specific gravity of 

 the accessible crust. Denser masses than those familiar 

 to us in the crust thus seem to form the great body of our 

 planet ; and it is very likely that our continental floors 

 are really portions of the lightest layer on the globe. 

 Processes of denudation, acting on the surface, have 

 separated the constituents of this layer; have collected, for 

 example, the heavy iron-ores at some points, or have 

 formed carbonates and sulphates and hydrous compounds, 

 of various densities, at others ; while heavier materials, 

 forced up through fissures from below, have added sheets 

 of basalt or bosses of gabbro to the manifold rocks of the 

 outer film. Nor must we forget that the remelting of the 

 old crust has locally enabled it to absorb masses above it, 

 and has thus increased its mineral complexity. The 

 general mass of the " floor," however, has remained much 

 as it was — a series of granites and gneisses and highly 

 siliceous schists of comparatively low specific gravity. 



We must refer in conclusion to Mr. Osmond Fisher's 

 " Physics of the Earth's Crust " ■ for a discussion of how 

 this light siliceous layer is probably thicker beneath the 

 continents and thinner beneath the oceans. Both the plumb- 

 line and the pendulum tell the same tale. The former should 

 be drawn out of the perpendicular by the attraction of high 

 continental land ; and from a survey of the mass of land 

 that stands, in any case, above the level of the sea, 

 the theoretical amount of deflection of the plumb-line can 

 be calculated. But the actual deflection has been found, by 

 experiments in India, to be less than the calculated amount. 

 Archdeacon Pratt, after much labour, arrived at this con- 

 clusion ; and Sir George Airy, in 1855, pointed out its pro- 

 bable explanation. The attraction of mountain-masses, and 

 consequently of continents as a whole, is deficient, because 

 the light crust is actually thickened beneath them ; hence, 

 for every great anticlinal ridge or bulge upon the surface 

 a corresponding ridge or bulge seems to be formed down- 

 wards, displacing the more dense and basic matter below. 

 Mountains have " roots," therefore, and tablelands are 

 similarly thickenings of the light outer crust. If there is 

 even a thin liquid layer — to make the smallest demand — 

 beneath the consolidated crust, it is easy to see how lateral 

 pressure in the crust may produce a bulge in two directions, 

 both upwards and downwards. The continental floor, on 

 these grounds, becomes still more real to us, and may be 

 compared to the mass of concrete on which buildings are 

 floated in equilibrium when foundations have to be laid in 

 oozy mud or sand. The formation of these knots in the 

 crust need not be opposed to our view of the instability of 

 continents and ocean-basins ; for the lower layers of a 

 continental mass may become melted off, in accordance 

 with Mr. Fisher's own "theory of the earth," while the 



* Second edition (1889), pp. 124, 195, 204, etc. 



thinner ocean-floor may become thickened in its turn by 

 compression. Most of us, however, must be content to 

 return from these somewhat speculative regions to the 

 continental floor itself; and in the relations of the rocks 

 that form it, in their mode of consoUdation, their inter- 

 penetration, and the deformations sufi'ered by them, we 

 shall find absorbing problems for a lifetime. 



ECONOMIC BOTANY. 



By .John R. .Jackson, .\.l.s., etc., Keeper nf the Museums, 

 Roijal (iardens, Kew. 



INTRODUCTOEY. 



THE first and by far the most important attempt, 

 in this and perhaps in any other country, to 

 elucidate and make popular the economic side of 

 botanical science was begun by the late Sir W. .1. 

 Hooker, when in 1847 one room of the building 

 now known as Museum No. 2 in the Royal Gardens, Kew, 

 was fitted up for the purpose to which it has ever since 

 been devoted. 



The foundation and progress of the collections now 

 contained in the three Museum buildings in the Royal 

 Gardens is certainly remarkable. It was in the year just 

 mentioned that the building, which had hitherto been 

 used partly as a storehouse for fruit, " was added by 

 command of Her Majesty to the Botanic Garden proper." 

 The nucleus thus formed consisted of the Director's 

 private collections, presented by himself. To quote from 

 the official guide to the Museums ; "No sooner was the 

 establishment and aim of the Museum generally made 

 known than contributions to it poured in from all quarters 

 of the globe, until in a few years the ten rooms of the 

 building, with its passages and corners, were absolutely 

 crammed with specimens. Application was therefore 

 made to Parliament by the Chief Commissioner for a 

 grant to defray the expense of an additional buildini; for 

 the proper accommodation of the objects, and the house 

 occupied by Museum No. 1, opened to the public in the 

 spring of 1857, is the result." 



From that time the collections have gone on increasing 

 in importance and value till at the present time they 

 stand unrivalled all the world over. Besides this, in 

 almost every botanic garden at home and abroad, as well 

 as in most teaching centres and in large towns, museums 

 on the system of those so well known at Kew have been 

 established. 



The result of all this has been the diffusion of a 

 knowledge of economic botany, so that at the present 

 time the subject is taken up even by our elementary 

 schools, most of which have their own small collections 

 for teaching purposes. It must be confessed, how- 

 ever, that until the last ten or twelve years the subject 

 did not command that attention its great importance 

 deserved. The structure of plants, their affinities, their 

 geographical distribution, and similar points attracted the 

 attention of the scentific worker, who gave no consideration 

 to their properties and uses. The connection, however, 

 between the purely scientific and the economic sides is 

 very apparent upon a moment's consideration. Thus, in 

 some natural orders there is a distinct property running 

 through the plants which constitute the order, which may 

 serve as an indication of their botanical affinities and also 

 prove them to be of economic value or otherwise. Such, for 

 instance, we find in the Malriiceie, where the inner barks 

 for the most part abound in long soft fibres, and the roots 

 and fruits of many are mucilaginous — the roots of the marsh 



