350 BULLETIN' : ML'SEUM OF COMPARATIVE ZOOLOGY. 



Etelhem Dr. Henry Munthe collected this species ^ on the summit of 

 one of the knolls of the Ascoceras limestone \\'hich are so abundant in 

 that region, while on the summit of another knoll of the same region 

 I collected Conchidium conchidium (Dalman), C. biloculare (Linne) 

 in the midst of a coralline limestone of the Ascoceras type, and on 

 Storo Karlso Pentaracrus estonus Lindstrom and Conchidium conchi- 

 dium occur in association in the midst of coralline limestones belonging 

 to the rocks of the highest part of the island. Orthis rustica Sowerby 

 is another species which can be found far above its natural strati- 

 graphic position in the midst of the highest rocks of the island, the 

 writer having collected it in clay pockets in the coral reef at Hoburgen. 



It has been quite generally considered that the highest rocks of 

 Gotland are the youngest, yet we find therein species whose natural 

 stratigraphic positions are lower in the section. I believe that the 

 high coralline rocks of Gotland are not the youngest of that island, 

 that they are older than any rock (there are a few exceptions) which 

 lies at the same level on their flanks, and that the coralline rocks 

 should be correlated with strata holding a much lower level. A recent 

 letter from Dr. Munthe shows that he is coming to the same conclusion 

 for he says, writing under date of October 2, 1915, "Last summer I 

 have ascertained that the 'Marl Shales' (Lindstrom's bed e) are 

 synchronous with a part of the reef-limestone ('Ascoceras limestone 

 in part'j in Garde etc." I believe the conditions are something like 

 those represented in Figure 1. 



Conditions like these make local correlation extremely difficult, 

 and when one considers that collections have been made without a 

 recognition of the possibility of their occurrence, it is readily seen to 

 what extent extra-Gotland correlation would be complicated. It is 

 believed that in this way are explained a great many of the various 

 difficulties which have arisen in connection ^nth the Gotland section. 



Assuming, then, that the corals stood above the bottom and rose 

 to or nearly to the surface of the water, it follows as a consequence that 

 during the period of the development of the reef that it grew upward 

 much more rapidly than the sediments accumulating around it. 

 Later, however, when the reef reached the surface of the water its 

 rapid growth ceased,^ and sediments accumulated around it far more 

 rapidly than it grew upward, the coral reef providing through its 



1 Personal communication. 



-This statement assumes that there was no sinking of the sea-bottom. If the sea-bottom 

 were sinking, growth upward would continue until the downward movement ceased, Eifter 

 which the conditions described would obtain. 



