5i8 



NATURE 



[March 29, 1894 



fectly understand the simplicity of the problem of drawing 

 the shortest line on any given convex surface, such as the sur- 

 face of this block of wood (shaped to illustrate Newton's 

 dynamical theory of the elliptic motion of a planet round the 

 ?nri) which you see on the table before you. I solve the 

 problem practically by stretching a thin cord between the 

 two points, and pressing it a little this way or that way with my 

 fingers till I see and feel that it lies along the shortest distance 

 between them. And now, when I tell you that Liouville has 

 reduced to this splendidly simple problem of drawing a shortest 

 line (geodetic line it is called) on any given curved surface 

 every conceivable problem of dynamics involving only two 

 freedoms to move, I am sure you will understand sufficiently to 

 admire the ;, -eat beauty of this theorem. 



The doctrine of isoperimetrical problems in its relation to 

 dynamics is very valuable in helping to theoretical investigation 

 of an exceedingly important subject for astronomy and physics 

 — the stability of motion, regarding which, however, I can 

 only this evening venture to show you some experimental illus- 

 trations. 



The lecture was concluded with experiments illustrating — 

 (i) Rigid bodies (teetotums, boys' tops, ovals, oblates, &c.) 

 placed on a horizontal plane, and caused to spin round on a 

 vertical axis, and found to be thus rendered stable or unstable 

 according as the equilibrium without spinning is unstable or 

 stable. , 



(2) The stability or instability of a simple pendulum whose 

 point of support is caused to vibrate up and down in a vertical 

 line, investigated mathematically by Lord Rayleigh. 



(3) The crispations of a liquid supported on a vibrating plate, 

 investigated experimentally by Faraday ; and the instability of 

 a liquid in a glass jar, vibrating up and down in a vertical line, 

 demonstrated mathematically by Lord Rayleigh. 



(4) The instability of water in a prolate hollow vessel, and its 

 stability in an oblate hollow vessel, each caused to rotate rapidly 

 round its axis of figure,^ which were announced to Section A of 

 the British Association at its Glasgow meeting in 1876 as results 

 of an investigation not then published, and which has not been 

 published up to the present time. 



GEOLOGICAL SURVEY OF 

 KINGDOM r 



THE UNITED 



II. 



Scotland. 



Leivisian Gneiss. — The most ancient rocks in the British 

 Islands, forming what is known as the Lewisian Gneiss, have 

 now been mapped continuously throughout the whole of their 

 extent on the mainland, from Cape Wrath to the Kyles of 

 Skye. They have been found to occur there in two distinct 

 conditions. Along the western borders of Sutherland and 

 Ross they form an irregular platform on which all later forma- 

 tions rest. The detailed work of the survey has brought to 

 light the fact that this platform had an exceedingly uneven 

 surface before the very oldest of the sedimentary formations 

 were laid down upon it. Mountains of gneiss from 2000 to 

 3000 feet high, with wide and deep intervening valleys, 

 already existed before the period of the Torridon Sandstone, 

 and were submerged beneath the waters in which that Sand- 

 stone was accumulated. But to the east of this primeval topo- 

 graphy, owing to the gigantic dislocations which have now been 

 traced for upwards of 100 miles from the northern shores of 

 Sutherland into Skye, large slices of the deeply buried gneiss 

 have been torn off and have been driven westward upon 

 fractured and crushed rocks of much later date. There are thus 

 areas of gneiss which have been moved and have undergone 

 much consequent internal rearrangement, while to the 

 west of these the old platform, still in great part covered with 

 the younger formations, has been left unaffected and reveals 

 the condition of the oldest rocks at the time when the earliest 

 of these over-lying formations was deposited upon them. 



The mapping of this region has shown the Lewisian Gneiss to 



1 Nature, 1877, vol. xv. p. 297, " On the Precessional Motion of a 

 Liquid." 



- Annual Report of the Geological Survey for the year ending 

 December, 31, 1892. By Sir Archibald Geikie, F.R.S., Director General. 

 From the Report of the Science and Art Department for 1892. (Some of 

 those portions of the Report wnich describe the scientific results of the 

 Survey operations during the last few years are reprinted here). (Continued 

 from page 497.) 



consist of what were probably masses of various deep-seated 

 igneous rocks, which, partly by segregation and intrusion, and 

 partly by subsequent intense mechanical deformation, have in 

 large measure acquired a gneissic structure. An order of 

 sequence has been made out among the more marked types of 

 erupted material, and it has been further ascertained that the 

 structures superinduced by crushing have taken place at 

 successive periods of great disturbance. 



Some of the most important observations in the area of the 

 ancient gneiss are those made in the Loch Maree district. 

 Mr. Clough has found there a group of rocks quite unlike the 

 u.'-ual types of the Lewisian series. They consist chiefly of fine 

 mica-schist, quart/.-schist, graphite-schist, and limestone, and 

 may be altered sedimentary rocks. If such should prove to be 

 their origin they will possess a special interest as being by far 

 the most ancient vestiges of detrital deposits yet detected in this 

 country. The relation of these rocks to the normal types of 

 gneiss around them have not been very satisfactorily 

 determined. 



In the course of the examination of the old gneiss where it 

 lies undisturbed below the unmoved Torridon Sandstone, the 

 officers of the Survey have ascertained that it had undergone 

 successive disruptions and much mechanical deformation before 

 the deposition of that Sandstone, that in short it had already 

 acquired all its present structure and had been irregularly and 

 deeply laid bare by denudation. We are still unable to say 

 how far the earliest foliated arrangement of the gneiss may be 

 due to movements such as those of flow-structure within a 

 plutonic magma, in which the component minerals have segre- 

 gated out. But there can be no doubt that after any such early 

 structure had been established other structures were superinduced 

 upon the gneiss by subterranean movements. Evidence of 

 these disruptions and of their effects has now been accumulated 

 over the whole area of the mainland. 



Torridonian. — The striking mass of chocolate-coloured sand- 

 stones, which enters so prominently into the scenery of the 

 west of Sutherland and Ross-shire, has now been mapped 

 throughout its extent on the mainland, with the exception of a 

 small area in the west of the latter county which remains to be 

 completed. Like the far inore ancient gneiss on which these 

 strata rest with so marked an unconformability, they are met 

 with in two distinct conditions. To the westward, where they 

 have escaped from the great dislocations already referred to, 

 they lie in almost their original undisturbed positions, insomuch 

 that one can hardly at first realise that their relative antiquity 

 can be so great as it demonstrably is. They resemble portions 

 of the Old Red Sandstone with which at first they were identi- 

 fied, and this resemblance extends even into the practical uses 

 that may be made of them. Along many parts of the West of 

 Sutherland and Ross-shire the thick bedded chocolate-coloured 

 freestones would furnish an excellent building stone in practically 

 unlimited quantities. 



An important group of shales has been found to occupy a 

 prominent place towards the base of the Torridon Sandstone 

 in Western Ross-shire. This group has now been followed to 

 the sea-coast, and has been found by Mr. Clough to attain a 

 still larger development in the southern part of the island of 

 Skye. They there contain thin bands of impure limestone, and 

 one of their members of much interest, forming only a thin bed, 

 consists largely of grains of magnetite and zircon. A diligent 

 search for fossils has recently been made by Mr. A. Macconochie 

 in this lower shaly group of the Torridon Sandstone, but 

 hitherto with scarcely any success, certain doubtful track-like 

 markings being the only indications of possible organic remains 

 which have been met with. 



In mapping the Applecross district, where the Torridon 

 Sandstone rises into an imposing group of mountains, Mr, 

 Home has encountered some singular volcanic orifices on a sand- 

 stone plateau about lodo feet above the sea. Two small 

 " necks " which rise there through the Torridon rocks, are filled 

 with blocks of the sandstone mingled with occasional bombs 

 of basalt, the whole being set in a dark green and grey paste 

 of similar materials. The fragments of sandstone have been 

 subjected to considerable alteration, for they have a glazed 

 aspect, while their quartz-grains have acquired a milky opalescent 

 or blue tint. There is no indication of the age^ of these two 

 volcanic vents, but they may with some probability be assigned 

 to the widespread Tertiary series which has left such prominent 

 memorials in the opposite island of Skye. 



During the past year in the district between Loch KishOrn 



NO. 1274, VOL. 49] 



