124 THE JOURNAL OF GEOLOGY. 



pletely crystalline schistose or gneissic character, but for the 

 most part the changes in the Upper Huronian rocks are those of 

 cementation and metasomatism. As with the Lower Huronian 

 areas of intense plication, they are sometimes but short distances 

 from those in which the rocks have been merely tilted. 



How deep the Upper Huronian denudation went it fs impos- 

 sible to say. We only know that at a maximum, the Upper 

 Huronian rocks are now 13,000 feet thick, and in certain other 

 places are entirely absent, the higher members disappearing first 

 and the lower members last. Thus the difference of the Upper 

 Huronian denudation is measured by 13,000 feet. To this must 

 be added the unknown thickness of the Upper Huronian rocks, 

 which have been wholly swept away, and the thickness of the 

 Lower Huronian and Basement Complex, which were cut at this 

 time. The thickness represented by these three elements is 

 unknown, but it is probably great. 



Of the outer limits of the Upper Huronian transgression, we 

 are as ignorant as of the preceding ones, but certain it is that it 

 had an extent to the outer areas mentioned as belonging to this 

 series. Beyond these limits no knowledge is available. The 

 original extent to the east, south and west of the Upper Huronian 

 will probably never be determined, since the ancient rocks are 

 covered by the Cambrian and post-Cambrian sediments. Whether 

 the transgression extended over the Great Northern area of 

 Canada to the Paleozoic deposits will doubtless be ascertained 

 when this vast region is studied in detail. 



The Kezveenazvan. — Again a change of conditions occurred, 

 and a great flood of basic volcanics, in beds of enormous thick- 

 ness were poured out. Later these were followed by more thinly 

 bedded volcanics. At about the same time a portion, at least, of 

 the Lake Superior region became immersed in the sea, since in 

 places the basement lavas of the Keweenawan are interstratificd 

 with sandstone and conglomerates. 



The Keweenaw series is composed lithologically of gabbros, 

 diabases, porphyrites, am3'gdaloids, felsites, quartz-porphyries, 

 etc., and of sandstones and conglomerates. The basic and acid 



