REPORT OF THE DIRECTOR 1 9 ID 99 



Keweenawan oi the Lake Superior region; but if, with Professor 

 Hogbom, the Seve group is considered upper Jotnian it might fur- 

 nish a parallel with the Manhattan schist and Invvood limestone of 

 southeastern New York. At all events the Seve group as repre- 



o 



sented by the Are schists is lithologically very similar to the latter. 

 The Grenville on the other hand is much more like the sedimentary 

 rocks in the Archean, than like the Jatulian. 



As a parallel with the Dala-porphyries we have the Precambric 

 volcanic rocks which are so generally distributed along the Atlantic 

 seaboard (9) ; which have been weir described in Wisconsin (10) ; 

 and which are of special interest in southeastern Missouri (11). 

 We can only say that porphyritic and felsitic rocks were also de- 

 veloped on this side of the ocean. 



The Jatulian (pronounced Yatulian) series was named originally 

 by Professor Sederholm of Finland. The exposures which pri- 

 marily suggested its establishment are found in the latter country. 

 They consist of quartzites, schists, dolomitic limestones and, strange 

 to tell, of beds of anthracitic carbon, which may attain a thickness 

 of two meters (12, p. 10). Strata referable to the JatuHan are less 

 abundant in Sweden and are in fact practically limited to one area, 

 the west side of Lake Wenern in the southwestern portion of the 

 country. They constitute a series of folded and metamorphosed 

 sediments, long known as the Dal- formation. 



The sub-Jatulian surface, or the one beneath the Dal-formation, 

 is believed by Professor Tornebohm to have been a mountainous 

 one, with valleys cut quite deeply, but Professor Hogbom considers 

 it to have been at most hilly. There is thus a time-gap represented 

 by erosion, but not of so great length as either the sub- Jotnian or 

 sub-Cambric. 



Back of the Jatulian lies the vast complex of extremely difficult 

 rocks forming the Archean of the later Swedish geologists. The 

 term is not used by them exactly in either the original sense of Pre- 

 cambric as given by Professor Dana, nor the later modification to 

 which it has been subjected by the Lake Superior group of geolo- 

 gists, as embracing only those rocks which were older than recog- 

 nizable sediments; but it applies to the great complex of ancient 

 rocks, consisting either of deep-seated intrusives or of sediments 

 always heavily metamorphosed. On the bases of the characters 

 shown by the foundation rocks on which the Jatulian rests, Pro- 

 fessor Hogbom infers an erosion of some thousands of meters and 

 therefore the greatest time-break in the history of the earth. 

 4 



