400 On American Geological History, 



The actual absence of animal life in the so-called Azoic Age 

 in this country is rendered highly probable, as Foster and AVhit- 

 ney show, by the fact that many of the rocks are slates and 

 sandstones, like fossiliferous Silurian rocks, and yet have no fossils • 

 and moreover, the beds on this continent were uplifted and folded, 

 and to a great extent crystallized on a vast scale, before the first 

 Silurian layers were deposited. A grand revolution is here indi- 

 cated, apparently the closing event of the early physical history 



of the globe.* 



(To he continued.^ 



* Foster and Whitney observe, (loc. cit. pp. 7, 26, 132,) that at Chippewa 

 Island (in the Menomonee River, near 45^'^ N., 88° W,,) the Potsdam sand- 

 stone Hes on the up-turned Azoic slates. At White Rapids, lower down 

 the stream, the same sandstone rests on the tilted edges of the Azoic quartz 

 rock. Near Presqu'Isle (not far from 46" 30'— 46? 35' N, 87.° 83^ W.,) a 

 similar contact of the nearly horizontal Potsdam and the vertical quarts 

 rock is seen. 



The Azoic of this continent was well studied and defined at a still earlier 

 date by the distinguished geologist of Canada, Sir William E. Logan. In 

 his Annual Report for 1846-1847, and that for 1848, he points out several 

 examples of the Silurian covering the contorted Azoic, and his subsequent 

 surveys have added to the facts of this kind. They occur north of the Lakes 

 Huron and Superior, and along and to the north of the Saint Lawrence. 

 Moreover, in the vicinity of the lakeg just mentioned, he found the Azoic 

 divided into two unconformable groups, a lower, since called by him the 

 Laurentian, and an upper, the Huronian ; the former consisting of granite, 

 syenite, gneiss, hornblende rock, hypersthene rock, crystalline limestones, 

 <fec. ; the latter of diorite, slates, white and red sandstones, conglomerates, 

 limestones, the whole much intersected by trap and metalliferous veins 

 containing native copper, (fcc, and having a thickness in some places, 

 probably of 9,000 to 12,000 feet. 



Sections representing the nearly horizontal Lower Silurian overlying the 

 Azoic, as observed by him in the vicinity of the St. Lawrence north-east of 

 Lake Champlain, are figured in the Quarterly Journal of the Geological 

 Society of London, for 1852, pp. 203 and 206. 



In the progress of the Geological Survey of New York, commencing in 

 1836, the fact that the crystalline rocks of Northern New York were older 

 than the Silurian was early shown, but good sections illustrating the super- 

 positions of the two were not given. 



At the meeting of the American Association at Cincinnati in 1851, when 

 Foster and Whitney first presented their views on the Azoic, Prof Mather 

 stated that he had traced the continuation of the system nearly to the 

 sources of the Mississippi and on the waters of the St. Peters, — a region since 

 reported on by Dr. D, D. Owen, (Geol. Survey of Wieconsin, Iowa and 

 Minnesota, 4to, 1852); Dr. H. King contributed observations on the Azoic 

 or iron mountain region of Missouri, (p. 194, Amer. Assoc. Rep. 1851,) 

 indicating the inferiority in position of these rocks to the Silurian, as had 

 been urged by Messrs, Foster and Whitney from the investigations by Mr. 

 Mersch under their direction ; and Dr. Engelmann described related rocks 

 in Arkansas between Little Rock and the Hot Springs. 



Professors W. B. and H. D. Rogers refer to Azoic Rocks as found in the 

 Appalachians ; but no instances of the superposition of the lowest Silurian 

 in those regions on other non-conformable beds have yet been published ; 

 and it is a question whether the metamorphic rocks are all related to those 

 of New England in age, or partly of tliis era of metamorphism and partly 

 Azoic. 



