284 Reports and Proceedings. 



an equally marked difference in the organic contents of the two formations. He con- 

 sidered the Purbeck formation to have been deposited in a lagoon subject to 

 occasional invasions of the sea, while the Wealden was in fact a large delta. Though 

 both were of freshwater origin, they were deposited under totally different conditions. 



The Chairman, alluding to the pseudomorphs of salt mentioned by the author, 

 stated that they had been somewhat compressed, and thus modified in form. They 

 had also been found in other beds in the Wealden. He commented on the extension 

 of the "Wealden strata even to the south of Moscow. In the Oxford and Buckingham- 

 shire area there was evidence of great denudation of the Purbeck and Wealden beds 

 prior to the deposit of the Neocomian, so that great changes would seem to have 

 taken place, giving rise to a great amount of denudation towards the close of the 

 Wealden period. 



Mr. Meyer agreed with Mr. Godwin-Austen and other speakers as to there having 

 been a certain amount of denudation of the Upper Wealden beds prior to the deposit 

 of others upon them, but this he regarded as merely local. It was the absence of 

 shingle rather than of gravel to which he had alluded in his paper. He thought that 

 there was a distinction to be traced between the Neocomian of the north of England 

 and that of the south, and that the middle beds of one were eqmvalent to the lower 

 beds of the other. 



II.— April 10, 1872.— His Grace tlie Duke of Argyll, K.T., "F.E.S., 



President, in the Chair. — The following communication was read : — 

 "Notice of some of the Secondary Effects of the Earthquake of the 

 10th January, 1869, in Cachar." Communicated by Dr. Oldham, of 

 Calcutta, with remarks by Eobert Mallet, Esq., C.E., F.E.S. 



This earthquake was a severe one, being strongly felt in Calcutta, 

 distant from the meizoseismic area about 200 miles, and far into the 

 plain of Bengal. 



The effects were examined on the spot a few weeks after the 

 shock by Dr. Oldham, who anticipates being able to fix the position 

 and depth of the centre of impulse by following the same methods 

 as those first employed by Mr. Mallet with respect to the great 

 Neapolitan earthquake of 1857. 



These results have not yet been received ; but Dr. Oldham has 

 forwarded an extremely interesting letter on the circumstances of 

 production of very large earth-fissures, and of the welling up of 

 water from these, derived from the water-bearing ooze-bed, upon 

 which reposed the deep-clay beds in which the fissures were formed. 



Dr. Oldham rightly views all these fissures, which were all nearly 

 parallel to and not far distant from the steep river banks, as 

 " secondary effects," and not due to fractures produced by the direct 

 passage of the wave of shock. He also shows that the welling up or 

 overflowing of the water in the fissures was a secondary effect also, 

 and negatives the notion entertained on the spot of mud-volcanos, 

 etc., having originated at those fissures. 



The chief aim of Mr. Mallet's remarks was to point out the 

 importance to geologists of rightly comprehending the dynamics of 

 production of these phenomena, and to show that the older notions 

 of geologists as to earthquake-fissures are untenable. He explained 

 clearly, aided by diagrams, the train of forces by which the elastic 

 wave of shock, on passing out of the deep-clay beds where these 

 have a free side forming the steep river banks, dislodges certain por- 

 tions and throws them off towards that free side — and that this is 

 but a case of the general law in accordance with which such elastic 



