fune 19, 1884] 



NATURE 



177 



nature of the surface motion at an earthquake observatory. 

 But this of itself tells us nothing of the speed and direc- 

 tion of transit of the disturbance, particulars which are 

 only to be learnt by connected observations made at several 

 stations. Any one earthquake, as a whole, lasts far too 

 long and begins too gradually to admit of the measure- 

 ment of time-intervals between its arrival at different 

 points, but if we can identify any single vibration in the 

 records given at several stations — spread over a moderate 

 area, and connected telegraphically with each other — the 

 problem admits of a fairly easy solution. A recording 

 seismograph at each station will give a complete record 

 of the earthquake as it appears there, and if, during its 

 progress, time signals be sent from one station and 

 marked on all the revolving plates, it will be possible to 



determine the differences in time of arrival of the same 

 phase of the same wave at the successive stations in the 

 group. From this, if the stations be sufficiently numerous, 

 the speed and direction of transit, and even the origin of 

 the disturbance, may be found with more or less precision. 

 But all this depends on our being able to recognise at the 

 various stations some one wave out of the complex 

 records deposited at each, and, especially in view of the 

 curvilinear nature of the motion, it would be hazardous to 

 say without trial whether this can be done. To ascertain 

 whether it can be done, and if so to organise groups of 

 connected stations to carry out the scheme roughly 

 sketched above, should be the next step in observational 

 seismology. J. A. Ewing 



University College, Dundee 



NOTES ON A FEW OF THE GLACIERS IN 

 THE MAIN STRAIT OF MAGELLAN MADE 

 DURING THE SUMMERS OF 1882-83 IN 

 H.M.S. "SYLVIA" 



THE western part of the main Strait of Magellan, to 

 which my remarks are confined, lies between rugged 

 and abrupt mountains, of rock mainly crystalline, but in 

 parts of slate. 



The highest peaks are not over 4500 feet high, and the 

 height of the snow-line is about 2700 feet. The land is 

 cut up into small areas by numerous and tortuous channels, 

 and, on the southern side certainly, no large masses of 

 land exist. The mountain ridges are mostly sharp and 

 steep, and afford but little area for snow to lie in quan- 

 tities, but wherever a mountain slope is moderate, there 

 it accumulates, and forms nevi, which may or may not 

 descend to lower levels. 



From this it will be seen that the glaciers spoken of are 

 small, only one snow-field, the " Northbrook," being of 

 any size. Much larger glaciers of course exist in these 

 regions, but were not in my beat, lying either to the south 

 about Mount Darwin and Mount Sarmiento, 7000 feet 

 high, or to the north on the mainland bordering the 

 western channels. 



Some ice-masses are ridiculously small, one I remarked, 

 at the end of summer, on a ledge a little below a very 

 sharp ridge 2700 feet high, was not probably larger than 

 10,000 tons. It lay entirely bare of snow on the southern 

 or shady side of the ridge, and was of blue ice. 



It is evident that it is the enormous amount of the 

 supply of material which accounts for the existence of 

 glaciers from such small origins, and in fact the deposition 

 of snow is going on all the year round for the majority of 

 hours out of the twenty-four. The winds are eternally 



from the western quarter, are usually fresh, and, arriving 

 moist from the Pacific against the rampart of mountains, 

 rush up their western slopes into the colder regions, where 

 constant condensation takes place. During my stay — 

 about eight months — the summits of the higher snow- 

 fields (3500 to 4500 feet) were only seen twice or thrice, 

 so continually are the mists around them. 



The daily duration of rain at the water-level during the 

 Sylvia s stay of about eight months west of Cape Fro- 

 ward was eleven hours out of every twenty-four. The 

 quantity corresponded to a yearly fall of 180 inches. 



Though the mean temperature for the year is low, the 

 range, summer and winter, is very small, so that flowering 

 plants which grow on the borders of the glaciers and on 

 exposed hills perish in England, from inability to with- 

 stand the sudden changes and lowness of the winter 

 temperature. 



The inference would seem to be that a Glacial period 

 need not so much depend upon extreme cold as on an 

 unlimited condensation with an equable temperature, low 

 enough at moderate altitudes to form snow. 



The glaciers are nearly entirely devoid of erratic blocks 

 or surface moraines. Coming, as they do, over every- 

 thing, down a hill-side, there is seldom an overhanging 

 mountain to discharge blocks ; where there is, the rock 

 is so solid that the very slight changes of temperature (for 

 the sun has no power here) is not sufficient to disintegrate 

 it. Even the glaciers therefore that descend nearly to the 

 sea are quite clean and spotless to the very end. 



I could never make out any raised beaches, nor other 

 signs of former lower level of the land ; all the evidence 

 is the other way. No beaches exist at the water-level of 

 the present day. There is not enough sea in these con- 

 fined channels to wash away the land, even if it was of a 

 softer nature. The steep rocky mountain-sides dip clean 

 into the water nearly everywhere. Thick moss covers the 

 hill-sides wherever it can get a hold, so that it is not easy 

 to see the true contours of them, and a more experienced 

 eye than mine might perhaps detect a raised beach where 

 I have failed to do so. 



Glacier from Mount Wharton 



Mount Wharton, 4400 feet high, on the south shores ot 

 Long Reach, sends down what I consider a rather re- 

 markable glacier, despite its small size. 



The upper part of the mountain, of a tolerably gentle 

 slope, is of an area of about four square miles. This 

 terminates everywhere in steep precipices, over which in 

 different directions the blue ice, which can be seen lining 

 the edge, tumbles, and forms glaciers remanie's in hollows 

 at lower levels in several places. On the south-eastern 

 side only is a steep slope, down which, after a series of 

 ice-falls, a leg of glacier, one-third of a mile wide, and one 

 mile and a half long, extends to within 1 50 feet of the sea- 

 level, and a quarter of a mile from the shore. At its end 

 it abuts against a hill, and from the fact of the ground 

 sloping away on either side from this glacier leg, it 

 appears that this slope is a ridge, down which the glacier 

 comes, as it were, astride. Where it strikes the hill, 

 it divides, and sends a final short leg towards the sea on 

 either side of the peninsula formed by the hill. 



The slope of the lower part of the glacier is 15 , and it 

 is much crevassed, and squeezed into pinnacles and 

 ridges, so that, when tolerably clear of snow, it looks like 

 frozen waves. 



There is no moraine on it, and, wherever I could see, it 

 lies on the solid rock, but a few stones are carried along 

 at the bottom of the ice, and, at its end, where it abuts 

 against the hill, the latter is a mass of loose rounded 

 stones (very few angular ones), up to the limit occasionally 

 reached by the glacier, which is well and curiously marked 

 by a narrow belt of trees, growing on the edge of the 

 tumbled stone moraine. Behind them the hill is of solid 

 rock, bare or moss-covered (see illustration). 



