GEOLOGICAL EXPLORATIONS AND LlTEliATUllE— 1887. Hi 



Irving- proceeds to distniss the relations to one anotlier ot" tlie other supposed 

 Huronian areas in the Lake Superior region. With respect to the Marquette 

 area, he refers to the two views lield as to the age of tlie granites, etc., on 

 the sides of the llurouiau trough, explaining that Kinihall, linxiks, and 

 others regarded them as unconforraably beneath the bedded rocks, while 

 Wadsworth and Roniinger, among the later geologists, regarded them as 

 eruptive into the bedded series. Irving liimself tinds that the green schists, 

 which on account of their banding had always been j^laced with the frag- 

 meutal beds as part of the iron-bearing series, are cut l)y granite dikes, 

 whereas, on the other hand, the granites and their associated schists are 

 separated from other members of the stratiform series by unconformities and 

 by basal conglomerates containing great l)owlders of the underlying granites, 

 etc. This is explained by making a divisioii of the rocks of the district 

 into two series, an older one comprising the green schists, than which the 

 granite is younger and into which it is intrusive, and a younger series com- 

 posed of the bedded fraginentals and their associated rocks, into which the 

 granite never sends dikes, but to whose lower layers it has A'ielded pebbles 

 and bowlders and large quantities of finer detritus. 



The ujjper series is composed mainly of detrital rocks, of whose frag- 

 mental nature there can usually be no doubt, though in many cases the 

 rocks are somewhat sheared, and have had developed in them secondary 

 sericite. With the detrital rocks are bedded limestones, cherts, etc., that 

 are lielieved to have been originally chemical sediments, and interbedded 

 diabasic eruptives. This series is a unit among the formations of the Lake 

 Superior region, and is so similar to the originally descril)ed Huronian in 

 its litln)logy and in its structural relations with other overlying and under- 

 lying formations, that it may safely be correlated with this and l)e called 

 Huronian. Investigations in the other supposed Huronian areas lead to 

 the same conclusion, viz, that the Huronian "is a true sedimentar^- group 

 in origin, in volume, in chronological distinctness from other groups above 

 and below it. It is not only compayaUe, as to volume, with the ordinanly 

 recognized rock groups, it exceeds most of them; l)esides which it is sepa- 

 rated from the adjacent rocks by tremendous unconformities, rei)resentative 

 of immense lapses of time." (Pp. 3 70-3 7 L) 



