January 1, 1897.] 



KNOWLEDGE. 



ilUSTRATEp MAGAZINE 



JTERAT 



LONDON: JANUARY 1, 1897. 



CONTENTS. 



The Recent Earthquake. Bv Prof. J. Logan Loblkt.F.G.S. 

 The Dawn of a Brilliant Era. By JoHX Mills, F.K.A.S. 



(Illustrated) 



Illustrious Philosophers of a Bygone Era. {Plate.) 

 On Describing and Drawing Insects. B.v Feed. Exock, 



F.L.S., F.E.S. {Illustrated)... 



The Spectrum Top. By T. L. Algek, LL.D., Ph.D. 



An Old Record of the Corona. By E. "Walter llArNDEB, 



F.R.A.S. {Illustrated) 



Evidence of the Evolution of Stellar Systems. By 



Isaac BoBEBTS, D.Sc, F.E.S. {Plate) 



Reports of Observatories 



Letters : — GEETErDE Buefobd Eawlujos ; J. FEENcn ; 



W. F. Llotd ; Feed. Exock 

 Literary Supplement — 



Que Book Table 



Eetiews of Booes. {Ulastrated) .. 



Shoet Notices ... ... ... ... 



Science Notes 



Books Received „ 



The Age of Mountains.— I. By Prof. J. Logan Loblet, 



F.G.S. {Illu.ifraied) 



Bird-Songs in Winter. By Chaelks A. Witchell 



The Decay of Clouds. By Dr. J. G. McPheesok, F.R.S.E. 



Protected and Unprotected Warships. By Xautilus. 



{Illu.ftraied) 



Animal Life in Arctic Regions. By F. W. Headlet, M.A. 

 The Face of the Sky for January. By H.Sadlee, F.R.A.S. 

 Chess Column. By C. D. Locock, B.A.Oxon. 



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THE RECENT EARTHQUAKE. 



By Prof. J. Logan Lobley, F.G.S. 



THE British Islands have been so free, during 

 historical times, from those violent seismic dis- 

 turbances that have from time to time devastated 

 other lands, that their inhabitants have very 

 generally come to the comforting conclusion that 

 this portion of the earth's surface enjoys an immunity 

 from surface movements. And yet even modern history 

 does not favour this roseate view, for, not to speak of the 

 earthquake shock that so alarmed our ancestors two 

 centuries ago, and others in the early part of the present 

 century, the very recent Essex earthquake, when a spire 

 at Colchester was thrown down and walls were fissured, 

 sufficiently shows that this part of the world is, like all 

 others, affected by the agencies of nature that are un- 

 ceasingly working beneath the exterior of the globe. 

 These agencies are very commonly all thought to be 



volcanic, and it is a dictum often stated as indisputable 

 that an earthquake is an uncompleted effort of nature to 

 establish a volcano. Such a statement, however, appears 

 to me to be not only misleading, but even the opposite of 

 the truth. 



A movement of the surface rocks of the globe may be 

 produced by several causes, and, undoubtedly, volcanic 

 action is one of those causes. But a volcanic earthquake 

 takes place in volcanic regions where volcanic activity has 

 successfully asserted itself and a volcano has been 

 established. Thus the earthquakes of volcanic regions are 

 consequent upon volcanic action and not the cause of it. 



But the causes of the earthquakes of non-volcanic 

 regions, such as are the British Islands in this epoch, are 

 non-volcanic, and must be consequent upon forces that are 

 acting independently of volcanic action. 



The surface rocks of the globe in every region, volcanic 

 or non-volcanic, it is obvious must be affected by those 

 cosmic agencies that disturb the normal condition of their 

 foundations— the underlyinir rocks. Increase and decrease 

 of temperature, causing expansion and contraction, are 

 such disturbing agencies ; and increase of temperature is 

 produced by chemical action, which may be commenced, 

 stimulated, retarded, or suppressed by decrease or increase 

 of pressure, resulting again from meteorological or sub- 

 terranean varying conditions. Earthquakes must con- 

 sequently be considered to be merely the expression at 

 the surface of deep-seated distui-bances, the normal results 

 of normal cosmic agencies. 



It is, therefore, a matter almost for wonder that earth- 

 quakes are not more frequent than they are in this and 

 other non-volcanic regions, and their rarity here and the 

 absence of violent or destructive shocks must be attributed 

 to the at present favourable geological conditions of these 

 islands. The earthquake of December 17th last affected 

 England along the strike of the Upper Paleozoic and the 

 Secondary rocks, the older Cambrian and Silurian areas 

 escaping, and also the Essex Tertiary area that suffered 

 in 1883. 



So far from these English earthquake shocks causing 

 alarm or terror, or apprehension for the future stability of 

 our country, they should have a reassuring effect, since 

 they show that the present geological structure of our 

 island is in accordance only with the production of non- 

 devastating earthquake shocks. From the extensive 

 faulting and Assuring of the older rocks we must conclude 

 that seismic disturbances were violent and frequent in this 

 region in Palaeozoic times ; but since that far-distant epoch 

 surface movements have been less frequent, from which 

 we may legitimately conclude that in the British Islands 

 the geological conditions have become more and more 

 favourable to that surface stability expressed by the words 

 terra ftrma. 



It may be appropriate here to give briefly the conclu- 

 sions as to the cause of earthquakes that are stated more 

 at length in Knowledge of -July, 1895. page IGl. Earth- 

 quakes of non-volcanic regions, which have their centres 

 far away from any active volcanic vent, are primarily 

 caused by chemical action when allowed by favouring 

 local physical conditions, prominent amongst which is 

 diminution of pressure. This chemical action, though 

 not sufficient to produce rock-fusion, is, however, adequate 

 to cause some. heat — with, possibly, gases and vapours — 

 that is of necessity accompanied by local expansions 

 and succeeding contractions, which occasion deep- seated 

 sudden fractures, and so give rise from separate and dis- 

 tinct dynamic foci to earth vibrations, which at the 

 surface cause earthquakes, and earthquake shocks and 

 tremors. 



