415 



nitehj thin perfectly conducting screen. We could scarcely be sure 

 beforehand that the conditions are sufficiently satisfied even by a 

 scratch upon a silver deposit. The case of an ordinary spectroscope 

 slit is quite ditferent. It seems that here the polarisation observed 

 with the finest practicable slits corresponds to that from the less 

 fine scratches on silver deposits". 



With the last words Rayleigh refers to an observation by Fizeau, 

 who on scratching in a silver layer on glass perceived that the 

 transmitted light was polarized perpendicularly to the direction of 

 the scratch, if the width of the latter was töVö "i™- If t'l's width 

 however was estimated at rswo tn'ïi- t''e polarisation was in the 

 direction of the scratch, viz. the electric vibrations were chieflj' 

 perpendicular to it. With spectroscope slits the latter case does not 

 occur. 



If will be rememJKM'cd that Dr Bois and Rlbhns') found with a 

 wire grating a |)oint of invelsion for ultra-red light, just as Fizk.\u 

 observed with scratches. 



Geology. — "On the occurrence of nodides of mnmjanesi' in 

 mesozoic deep-sea deposits from Borneo, Timor, and Rotti, 

 their snjnificance und mode of formation" . By Prof. G. A. 



F. MOLENGRAAI'T. 



(^Communicated in the meeting of January 30, 1915). 



The question whetiier deep-sea deposits, and more especially 

 oceanic abysmal deposits, of earlier geological ages, take part in 

 more or less appreciable degree in the formation of the existing 

 continental masses, may be considered of prime importance for the 

 solution of several geological problems. If answered in the affirma- 

 tive, Ihe conclusion al once follows that movements of the earth's 

 crust must have taken [)lace of an amplitude, sufficiently great, to 

 bring deposits formed at a depth of 5000 metres or more, above 

 the surface of the sea. 



Some twenty years ago the opinion prevailed, that true abysmal 

 deposits of former geological ages, had nowhere been proved, with 

 certainty, to exist in the continental areas. It must be admitted that 

 at that time, descriptions of occurrences of such abysmal deposits 

 were scanty and far from convincing. This may have been partly 

 caused by the fact, tiiat fossil deep-sea deposits are not conspicuous 



1) H. DU Bois and H. Rubens. Ber. Berl. Akadeniie 112'.). 1892, 



