STRATIGRAPHIC UNCONFORMITY IN TPIE HURONIAN. 101 



of the two characterizing "slate conglomerates." We have ourselves been 

 over the whole breadth of the system, in its typical region, -without at first 

 discovering any necessity for new interpretations. It was the study of con- 

 temporaneous formations in northeastern Minnesota and north of Lake Su- 

 perior which first clearly revealed to us the fact that the so-called " Huronian " 

 has there been rent into two systems. It was this revelation in northwestern 

 geology which prompted us to a review of the recorded facts in Lake Huron 

 geology, and to new explorations in certain critical districts between the 

 Lake Huron and Lake Superior districts. 



Premising that the present inquiry must necessarily be based on evidences 

 drawn from structural and lithological data, we present, first, the fact of the 

 stratigraphic unconformity between the upper and lower divisions of the original 

 Huronian, as described by its authors. We have already directed attention 

 to the fact that in the region around Lake Wahnapitse, northeast of Lake 

 Huron, the average dip of the members nearer the gneiss is much higher 

 than of the members remoter from the gneiss. As a rule the older members 

 there, as elsewhere, stand nearly vertical, while as a rule the newer members 

 very seldom have an inclination greater than 45°. But the region is one of 

 numerous abrupt and local disturbances. It hence becomes quite true that 

 in single instances the lower members, or what seem to be the lower mem- 

 bers, approach a horizontal position, while the higher members, or what seem 

 to be the higher members, are locally thrown into steep attitudes. In study- 

 ing the details of other regions, we find a similar contrast between the dips 

 of lower and higher members of the system. We do not consider this 

 method of reasoning conclusive ; but it seems to show that if the later dis- 

 turbances which affected in common all the strata embraced in the nominal 

 Huronian have left the older strata more highly tilted than the newer, the 

 result depended on the fact that they were originally more highly tilted — 

 that is, that the older and newer members of the system were in a state of 

 discordance before the disturbances occurred to which we here refer. 



Again, we have already cited Sir William Logan's statements in refer- 

 ence to the structural relation of certain Huronian members and the under- 

 lying gneiss along the north shore, and near the shore, of Lake Huron. 

 Here, he says, the slate conglomerate has as low a dip as nine degrees, while 

 the sandstone [granular quartzite] above it is nearly horizontal and the 

 underlying gneiss is nearly vertical. That this flat-lying slate conglomerate 

 is the "upper" is proved by the fact of a quartzite overlying it, for no 

 quartzite is described as overlying the " lower " slate conglomerate (see the 

 general section already cited from Logan). It is also proved by the facts 

 of its discordance with the underlying gneiss ; for in every instance in which 

 the lower slate conglomerate has been traced to the proximity of the gneiss, 

 these two formations seemed to be conformable in position, though generally 



XVI— Bull. Geol. Soc. Am., Vol. 2, 1890. 



