204 JR. D. Irving — Is there a Huronian Group ? 



Van der Plaats, giving more than sufficient credit to the 

 previous experiments on sulphuric acid, has suggested in a 

 note that the experiments were but few in number, were in 

 fact, but preliminary (provisoire) and that it would have been 

 better to produce the current of air by pressure. With the 

 connections used, whether a current be produced by excess or 

 defect of pressure is perfectly indifferent. The leakage at any 

 joint was proved to be less than a tenth of a cubic millimeter 

 in the time of an experiment. The experiments were also of 

 such number and character as to be final (at least for the pres- 

 ent), as to the question whether the use of sulphuric acid for 

 drying gases in the ordinary course of analysis or research in- 

 troduces any measurable error. 



Our knowledge of the behavior of the three principal drying 

 agents is now as complete as is needed at present. Dibbits 

 showed how much moisture is left unabsorbed by calcium 

 chloride at different temperatures. He also established the 

 difference between the amounts left unabsorbed by sulphuric 

 acid and b}^ phosphorus pentoxide. I have shown previously 

 that sulphuric acid leaves unabsorbed not far from a fourth 

 of a milligram of moisture in 100 liters of a gas. Now it 

 appears that the moisture left unabsorbed by phosphorus pent- 

 oxide, if capable of determination, may be very roughly stated 

 as possibly a fourth of a milligram in 10,000 liters. 

 749 Republic St., Cleveland, Ohio, May, 1887. 



Art. XXIV. — Is there a Huronian Group?* by R. D. Irving. 



Synopsis of Contents. 



I. Starting with a statement of the values of the terms System, Group and 

 Formation, as used ia the nomenclature recently proposed by the Director of the 

 United States Geological Survey, this paper enquires whether there can be carved 

 off from the upper part of the great complex of rocks ordinarily known as 

 Archasan, a Huronian series, entitled to rank with such groups as the Cambrian, 

 Silurian, etc. 



IT. The rock series of the north shore of Lake Huron, between the Mississagui 

 and St. Mary's rivers, mapped by Logan on Plate III of the atlas to the Geology 

 of Canada, 1863, is the original or typical Huronian, and no other. The enquiry 

 muse then begin with this region. 



III. This typical series of rocks is next shown to be entitled to the group rank 

 by (1) its intrinsic characters, and (2) its structural, and consequently its chrono- 

 logical separateness from the older Archaean, and younger Cambrian and pre- 

 Cambrian rocks of the region. 



IT. The so-called Archaean rocks of the Marquette, Menominee, and Penokee 

 districts of the south shore of Lake Superior are next considered, in the order of 

 the districts named, and are shown to present the same divisibility into an upper 

 detrital, and a lower crystalline schist member, which members are to be cor- 

 related respectively with the type Huronian, and with the crystalline rooks beneath 



* Read by invitation before the National Academy of Sciences, at "Washington, 

 D. C, April 22, 1887. 



