( 237 ) 



determine, in consequence of which the direct remedy of fixing the age 

 fails or at least becomes uncertain. Especially in Belgium (lie division of' 

 the upper- tertiary deposits has given rise to many contradictions during 

 a long time because the incomplete and indistinct fauna opposed the 

 question of the exact parallelism with similar strata in a high degree. 

 Only by the extensive researches of E. van den Broeck and others in 

 the Antwerpian pliocene, when those deposits were uncovered on a 

 large scale by the establishment of harbours and docks, this altera- 

 tion was recognized as a general secondary action which appears 

 everywhere where the circumstances of penetrable cover and a 

 situation above the level of the ground- water allow it. The limit 

 between the decomposed glauconite sand (yellow crag) and the 

 original sand (gray crag) is not at all to be considered as a geological 

 limit but denotes only the level of the ground-water. 



I have mentioned these well-known facts in order to arrive at 

 another observation which was made by inquiring the altered depo- 

 sits and for which an acceptable explanation was ready. It was 

 fixed that the resistance of the different shells against the dissolving 

 action was a different one and thus more and less resisting shells 

 were to be distinguished. The still recognizable stones and printings 

 belonging to the genera Terebratula, Ostrea, Pecten, etc. and these 

 shells being known to consist of calcite, while the small gastropoda 

 shells are composed of aragonite, it was concluded that calcite is 

 dissolved in muter containing carbonic acid with much more difficulty 

 than aragonite. In the "Handbuch der Pal aeon tologie" of Karl A. 

 von Ztttel (Volume II, page 12) for instance this is expressed as 

 follows : 



"Die Kalkspathschalen zeigen eine ziemlich betrachtliche, die 

 Aragonitschalen eine sehr geringe Widerstandsfahigkeit gegen die 

 auflösende Thatigkeit kohlensaurehaltiger Grewasser*. In Ablagerungen, 

 wo fast alle fossilen Muscheln oder Schneckengehause zerstörl und 

 uur (lurch Steinkerne angedeutet sind, findet man wohlerhaltene 

 Schalen von Ostrea, Pecten, Pinna, Trichites, u. a." 



I do not know whether this fact ever has been examined by 

 experiments upon the pure minerals. The failing of any communi- 

 cation concerning that subject in different mineralogical test-books 

 suggests that this has not taken place and thus it may be useful to 

 examine this different behaviour experimentally, so far as the difference 

 of solubility has an importance for the practical geology 1 ). 



') Some time after having finished lliis communication I got acquainted with an 

 css;iy of II. \V. IToote : Ueber die phj sicalisch-chemischen Beziehungen zwischen 

 Aragonit und Calcit (Zeitschrifl für physikalische Chemie, Band 33, pag. 740) to 



16* 



