592 EDWARD SAMPSON 



In this connection may be mentioned the radiolaria-free chert 

 occurring between the interstices of pillows both in Notre Dame 

 Bay and in pre-Cambrian areas of the Lake Superior district and 

 on the east side of Hudson Bay. 



Cherts Formed as Chemical Precipitates 



The field relations and the structure as revealed by thin sections 

 give evidence that the cherts are chemical precipitates. As to the 

 field relations, Van Hise and Leith, in their study of the original 

 iron-bearing formations of the Lake Superior region, have used a 

 criterion which may be applied with equal force to the sedimentary 

 cherts of Notre Dame Bay. This has been summarized by Leith 

 as follows: 



The sharp contacts of iron formation rocks, the great thickness and general 

 lack of contained chemical detritus in the iron formation, notwithstanding its 

 association with chemical deposits, leads us to believe that the deposition of the 

 iron formation sediments began suddenly and went on rapidly. It is not easy 

 to conceive of the total inhibition of the deposition of fragmental sediments 

 during this time, but relative rapidity of the deposition of the iron salts would 

 mask the fragmental deposition. This rapidity of deposition would be more 

 in accord with the hypothesis of rapid direct contribution of iron salts following 

 igneous outbreak than their more slow accumulation through normal erosion 

 processes of igneous rocks, and especially does this seem likely to be true 

 where the iron formations completely lack associated fragmental material and 

 are bounded both above and below by ellipsoidal flows.'' 



The silica of the cherts is very fine-grained, generally micro- 

 cry stalHne. In some sections there are areas in which exceedingly 

 fine specks of faintly birefringent siHca occur in an amorphous base. 

 The occurrence of this amorphous silica is consistent with the theory 

 that the cherts are chemical precipitates, and, as it is present in 

 sections which show distinct radiolarian remains, it is difi&cult to 

 explain as being formed of comminuted radiolarian tests. 



The evidence is clear that the chert of two previously described 

 localities at Fortune Harbor has been in a gelatinous condition. 

 Both the jasper from the interstices between the pillows on the 

 Northwest Arm (p. 584) and that from the heavy bed at the manga- 

 nese prospect on the Southwest Arm (p. 583) show concentric rings 



' C. K. Leith, Lake Superior Type of Iron Ore Deposits, in Types of Ore Deposits, 

 edited by H. F. Bain, San Francisco, 1911, p. 75. 



