62 



NA TURE 



{May 15, i i 



iron were unharmed by earthquake shocks which did 

 enormous damage to other buildings. This apparent 

 contradiction may perhaps be explained by supposing that 

 a lightly-built structure would give to every movement, and 

 a very massively-built structure would resist such move- 

 ment ; either would bear a considerable earthquake shock 

 unharmed, but buildings of intermediate resisting power 

 would be destroyed. 



Upon the twisting motion often noticed in earthquakes, 

 and especially referred to by Dr. Taylor as observed at 

 Langenhoe, Peldon, Fingrinhoe, and Abberton, much dif- 

 ference of opinion has existed. From the time of Aristotle 

 it has been commonly attributed to a vorticose movement 

 of the earth's surface at the part affected. The effects pro- 

 duced seem at first quite to accord with this explanation, 

 as when in Japan a chimney ten feet high, and two feet 

 by three, is broken in two, the upper half being twisted 

 diagonally round without other fracture or displacement ; 

 or when, at Mendoza, a church tower has its lower story 

 uninjured, its middle story turned through an angle of 

 nearly 90 , and its highest story thrown completely over. 

 But Mr. Mallet, who recorded similar cases in the Nea- 

 politan earthquake of 1 857, denied that this is in any 

 case the correct explanation, and he believed that the 

 twisting is best explained by resolved motions, due to the 

 transit rectilinearly of the shock. 



As regards the range of the recent earthquake very 

 little more is known than was recorded during the first 

 few days after the shock. The most important new fact 

 is that mentioned by Dr. Prince of the shock being ob- 

 served at Crowborough in Sussex ; this proves that the 

 Wealden area was affected, although the shock must have 

 been very slight. Probably if the Palaeozoic rocks were 

 as near the surface there as was once hoped the shock 

 would have been more distinctly felt. 



There can now be little doubt that the 'origin of the 

 shock was vertically under West Mersea or thereabouts, 

 and that the wave must have travelled in all directions 

 away from that area, but not necessarily with equal force 

 and rapidity in all directions. The observations as to the 

 direction of motion generally agree with this view ; but in 

 the neighbourhood of London there are some curious 

 differences. The observations in London itself generally 

 give an east and west direction, whereas some on the 

 north side of London appear to point to a more north and 

 south direction. 



No observations are recorded of the connection of the 

 earthquake wave with minor details of geological struc- 

 ture, such as the outcropping of certain hard or soft beds, 

 or with lines of fault. The only instance of the latter kind 

 known to me is at St. John's, near Greenwich, where the 

 shock was felt very close to a fault, well exposed in 

 the railway cutting just west of St. John's Station. Bui 

 another and perhaps better explanation is that the shoi 

 was there felt by an invalid lying quietly in bed, and very 

 sensitive to movement. 



In the map here given, an attempt has been mule to 

 mark the positions of all places at which the shock was 

 felt, so far as can be learnt from published accounts ; but 

 in Essex, Suffolk, and North Kent only a few of such places 

 could be marked. By marking the on 1 ops of the older 

 rocks (Carboniferous and earlier), the possible connection 

 of these with thetravel of the earth | lake w ive maybe seen. 

 This is made clearer by the section. The position of the 

 Palaeozoic rocks is known at Harwich and London ; there is 

 some uncertainty as to their position under Reading and 

 Colchester, but for the purpose intended, and regard 

 had to the depth at which the sho k mu ;l h ive originated 

 (certainly far within the Pal seozoi 1 1 . , the line drawn 

 is sufficiently near the truth. We can see how the shock 

 can have been propagated through the hard Palaeozoic 

 rocks and been felt where these are bare or thinly covered 

 with newer rocks, whereas through the thick and softer 

 Secondary and Tertiary rocks the wave might travel a 



shorter distance. Possibly also this section may suggest an 

 explanation of the double shock which was sometimes re- 

 corded : the first would be that travelling quickly through 

 the hard Palaeozoic rocks, the second that propagated 

 more slowly through the softer overlying newer rocks. 



W. TOPLEY 



VOLCANOES ON THE SHORES OF LAKE 

 NY ASS A, AFRICA 

 T^)R. LAWS, on his return to Europe from the mission 

 •*—^ station at the north end of Lake Nyassa, passed by 

 Naples, where I had the pleasure of meeting him. 

 Amongst other information that I gleaned was that 

 pumice-stone is very abundant in the locality above-men- 

 tioned and on the shores of the lake, where pebbles of 

 coal are also met with. He also informed me that many 

 of the rocks had a striking resemblance to the volcanic 

 tufas around Naples. Dr. Laws happened to have a 

 specimen of pumice, which he kindly placed at my dis- 

 posal. 



The specimen forms about two-thirds of a flattened 

 ellipsoidal pebble of about \\ X i\ X| inches in dia- 

 meter. It is of a dirty buff colour, darker in spots, the 

 result of oil stains in packing. The grain is fine; there 

 are very few large cavities, which are multilocular, with 

 smooth-walled spheroidal-shaped alveoli. The specific 

 gravity of the mass is light. The characters indicate 

 great homogeneity of material, only a moderate amount 

 of dissolved water in the original magma, and an eruption 

 of true paroxysmal type. A few minute crystals of 

 I m are discernible with the naked eye, and rarely 

 also a small black spot, which we shall see to be 

 pyroxene. 



It is easily sectionised, and when examined under alow 

 power, shows a remarkable uniformity of size in the pores. 

 Those near and opening upon the surface contain a few 

 diatoms indicating the action of water as the cause of the 

 pebble-like form. 



The magma is a perfect glass of light straw-colour. 

 Scattered through it are a few small irregular crystals of 

 sanadin, fairly clear, but of irregular boundaries in many 

 cases, as if they had wavered between crystallisation and 

 fusion. A few are twinned on the Carlsbad type, and a 

 few also present fine wavy striation parallel to their longer 

 axis. At one spot were two or three sanadin crystals 

 inclosing dark brownish-green pleochroic microliths, too 

 irregular to measure the angle of extinction, but which 

 looked very much like amphibole. There were to be seen 

 a few well-formed crystals of pyroxene of light pea-green 

 colour, quite free from pleochroism, and with characteris- 

 tic crystalline boundaries and cleavage, with absence of 

 inclosures, In the immediate neighbourhood of the large 

 were a few microliths of the same 

 mineral ; the average angle of extinction was 49°, and 

 ranging within narrow limits. No other " formed " 

 materials were discernible except a mass of dirty brown, 

 dusty matter involved in a group of sanadin crystals, 

 which might be magnetite. The whole character of the 

 specimen is strikingly like some of the basic pumices of 

 Monte Somma, and aim 1st indistinguishable from some 

 specimens of Phase VI. Period 1 [Quart. Journ. Geol. 

 Soc, January 1884). 



I regret that for want of a balance I have not been able 

 to analyse the specimen, though I am inclined to place it 

 amongst volcanic rocks containing less than 55 per cent, 

 of silica 



The specimen itself is in no way remarkable, but it is 

 interesting as indicating the existence of continental vol- 

 canoes some hundreds of miles from the seashore, although 

 in the immediate neighbourhood of a great lake, as also 

 an additional grain of acquaintance with the geology of 

 the mysterious interior of the " dark continent." 



My informant has promised to forward me a collec- 



