990 



bulletin: musei':m of comparative zoology. 



Summary. 



From the above summary of the sections in the principal Ordovician 

 belts of Sweden, certain facts should appear. 



It will be noted that the Dictyonema zone is present in almost all 

 sections, and that the character of the deposit is controlled by the 

 underlying beds, a fact which in itself is evidence of a cessation of 

 sedimentation, uplift, and erosion. In North Oeland, where the 

 basement beds are the Parado.vidcs tcssiui sandstone, an Obolus sand- 

 stone is developed, and in the northern belt, Dalecarlia-Gastrikland, 

 where the older strata are pre-Cambrian crystallines and Lower 

 Cambrian sandstones, one finds Obolus conglomerate and Obolus 

 sandstone. That the Obolus sandstone is of the same age as the 

 Dictyonema shale is of course abundantly proved by their interstratifi- 

 cation v.-ith each other at several places in Esthonia. In southern 

 Oeland, in Scania, and at Christiania, where the underlying strata 

 are the black shales and limestones of the Upper Cambrian, the Dictyo- 

 nema zone is developed as a shale without sand and has even, in places, 

 irregular lavers of limestone. 



Over a certain area, in Xerike and part of Vastergotland, the 

 Dictyonema zone is absent. Thus, at Kinnekulle the Ceratopyge 

 limestone rests upon the limestone and shale of the Upper Cambrian, 

 at Ekedalen the Planilimbata limestone rests directly on the Upper 

 Cambrian, and at Hunneberg the Ceratopyge limestone rests on the 

 Upper Cambrian limestone in most places, though in some spots about 

 three inches of shale belonging to the Dictyonema zone have been 

 reported. The arrangement of these localities free from Dictyonema 

 deposits suggest a large, low lying island composed of uplifted Upper 

 Cambrian strata, which was progressively submerged during Lower 

 Ordovician time, but not completely covered till toward the end of 

 Planilimbata time. Correlated with this may be the distribution of 

 the Lower Didymograptus shale. 



As has already been pointed out repeatedly by the Swedish geolo- 

 gists, when the Lower Didymograptus shales are present in any sec- 

 tion, the Planilimbata limestone is absent, and when the latter is 

 present, the former is absent. This has naturally led to the deduction 

 that the two formations were deposited at the same time and owe their 

 dissimilar faunas to the very different conditions of sedimentation. 

 On this point, the evidence, though strong, does not seem to be abso- 

 lutely conclusive. It would seem that, should it happen that lime- 



