( 236 ) 



Geology. — "On the behaviour of fossil shells in water containing 

 carbonic acid." By Dr. P. Tesch. (Communicated by Prof. 



S. Hoogewerff). 



(Communicated in the meeting of September 26, 1908). 



It is a well-known phenomenon that all strata which are more 

 or less calcareous and are situated above the surface of the ground- 

 water, when not consisting- of wholly impenetrable clay, are exposed 

 to a slow action of solution. The explanation of this extraction of 

 lime is easily given by the dissolving action of the penetrating rain- 

 water, by which the quantity of lime is gradually withdrawn from 

 the higher parts of the deposit and in some cases is concentrated 

 in the lower parts in marl-puppets (for instance in the loss), in 

 other eases "Jiowever is totally carried off (as a boulder-marl can be 

 transformed in a clay without any lime). The rain-water contains 

 already a good deal of oxygen and carbonic acid ; the part that 

 sinks away in the bottom still takes up the carbonic acid which is 

 formed in the upper-crust by the putrefaction of the vegetable rests 

 and thus it is enabled to exercise an oxidating action on the iron- 

 containing minerals and a dissolving action on the present lime. 



Especially the fossiliferous glauconilic sands are totally changed 

 by this alteration. In the first place the glauconite is dissected 

 and the iron which for a great part exists already in the terri-form, 

 is separated as linionite and forms a binding for the grains of sand. 

 The sand in the beginning of a dark or light green changes into a 

 yellow or brown sand, which in some cases is bound so strongly 

 that the name of a linionite sandstone may be given. Everywhere 

 where glauconilic sands are situated above the surface of the ground- 

 water and are not covered by protecting clay-beds, this phenomenon 

 can be observed. In the southern [tarts of the Lower Rhine basin 

 for instance the upper-oligocene sea-sands show everywhere where 

 they have kept their original niveau, a yellow or brown colour 

 by the disintegration and oxidation of the glauconite grains (heaved 

 block of Myhl between Hiickelhoven and Birgelen on the eastern 

 bank of the Roer, the sandquarries of Gerresheim, Grafenberg and 

 Rothenberg east of Dusseldorp etc.). In Belgium the "sables et gres 

 ferrugineux de Diest", the "Crag jaune d' Anvers" etc. are so con- 

 verted glauconitic sands. 



In the second place however the water with carbonic acid acts 

 as a solvent upon the lime shells of the fossils, which disappear 

 totally or rest as printings and stones and are more difficult to 



