260 PROCEKDI>^GS OP THE GEOLOGICAL SOCIETV. [Apiil 10, 



are some curious and as yet, to me, uuexplaiued cases of explosive 

 noises, like separate discharges of artillery, distinctly aiidiblc and for 

 some time at intervals, after the shock had ceased, but without any 

 accompanying (at least perceptible) motion. One man, an engineer, 

 declares he saw successive waves of undulation pass across his garden, 

 where he was sitting, and that the motion (rendered more distinct by 

 the regular vibration of the tlowers following the waving of the 

 ground) was in waves about 8 inches long and 1 inch high ! I 

 got at this by cross examination and making him first say what 

 he saw, then plotting the wave as he described it (he gave more than 

 double the height I have stated above at first), and gradually, by 

 making him come back again to the spot and again plotting the wave, 

 trying to get at the facts. That he saw the waves pass along as suc- 

 cessive ones he never swerved from ! 



• " The shake was a sharp one and no mistake, but by no means so 

 bad as people supposed at first. 



" I necessarily write in haste to catch this mail. You will excuse 

 my not writing before, as I only got your letter a day or two since 

 on my return, and am literally in the midst of a heap of writing and 

 papers — engulfed, in fact. 



" Yours, ever sincerely, 



" TnoMAs Oldham." 



Amongst the several points of scientific interest in Dr. Oldham's 

 communication we may single out for some remarks : — the mechanism 

 of the production of those great earth-fissures which he so well 

 describes ; that of the production of the mud-vomiting fissures or 

 holes, also described ; and the reiterated noises heard at intervals 

 after the shock, but not themselves attended by sensible movements. 



In all the older earthquake narratives the formation of earth- 

 fissures, large or small, and whether through rock or less coherent 

 material, is, so far as it is intelligibly attributed to any particular 

 play of dynamic forces, set down to be the direct result of the pas- 

 sage of the shock. 



The shock was felt to be some sort of undulatory movement of the 

 ground ; and the conclusion is jumped to that the ground heaved 

 into curved billows, translated from point to point of its surface, and 

 giving place to others, with short intervals betAveen, like the surface 

 of the stormy ocean, or like that of a carpet shaken upon air (as 

 John Mitchel, of Cambridge, described it) ; the earth's substance, 

 being stretched beyond its limits of extension or cohesion, was thus 

 bent into the convex curve of the terrestrial billow; and so the 

 rent or fissure opened ; and this closed again as the surface retracted 

 in falling back to the level or below it into the trough between two 

 of these billows. 



This is the crude notion of the direct production of earthquake- 

 rents or fissures, which, with as little of inquiiy as of hesitation, has 

 been handed down from book to book of even some of the leaders of 

 geology, and may stiU be found in some such works of very recent 

 date. The reception of this notion through the senses it was that 



