STRATiaKAPHICAL GEOLOGY. 3 



Mamainse, it would appear that the lower rocks of the latter series do not come 

 to the surface at Michipicoten Island, and that the higher rocks of the Michipi- 

 coten series have not been developed at Mamainse, or lie beneath the waters of 

 the lake to the southwest of the promontory. It would, therefore, appear just in 

 estimating the thickness of the upper copper-bearing rocks of the eastern part 

 of Lake Superior (which are Hui-onian), to add to the Mamainse series the above- 

 mentioned 4,000 feet of resinous traps, or porphyrites, which would make the 

 whole thickness at least 20,000 feet. (Geo. of Can., 1863, pp. 55, 67, 86 ; Geo. of 

 Can., 1866, pp. 132, 141.) 



In addition to the Eozoon canadense, only a few fossil species have been 

 described from the Huronian rocks, and these have been placed in genera of 

 uncertain affinity, to wit: Aspidella, Stenotheca, and Scenella. 



The metamorphic strata, equivalent to the Laurentian and Huronian series 

 of Canada arc described in Safford's Geological Survey of Tennessee, as many 

 thousand feet thick. Fossil remains of foraminifera have been found in them. 

 They have also been found in the metamorphic rocks of Europe; so that these 

 rocks are no longer called azoic. 



Next above the Archsean subdivision lies the great Silurian formation, first 

 determined by Sir R. I. Murchison, and named in memory of the ancient Silures, who 

 inhabited Wales, where he first studied the exposure of the rocks. He subdivi- 

 ded it into the Lower Silurian and Upper Silurian formations, and these names 

 have been adopted in this country. The Lower Silurian is much the most exten- 

 sive, and«is divided into groups or subdivisions, in ascending order as follows : 



1. The St. John's Group, consisting of black shales and sandstones, resting 

 conformably upon older schistose rocks, at St. John's, New Brunswick, 3,000 

 feet thick. (Geo. of Can., 1866, p. 235.) 



This Group of rocks contains the remains of Paradoxides, Conocephalites, 

 Arionellus, Miorodiscus, Agnostus, Orthis, and other fossil genera. The Para- 

 doxides beds near Boston are supposed to belong to it. 



2. The Potsdam Group, which takes its name from Potsdam in Northern 

 New York, where it is well developed, and consists of a fine-grained, even-bedded 

 sandstone, traversed by parallel joints. The Potsdam Group is sometimes divi- 

 ded into Upper Potsdam and Lower Potsdam Groups. The sandstones and 

 limestones on the north shore of Belle Isle, and the rocks which, in the State of 

 Vermont are called the Georgia slates, and the red sand-rock, belong to the 

 Lower Potsdam Group. The Upper Potsdam Group is displayed in Minnesota, 

 Wisconsin, New York, Pennsylvania, and other States. In Tennessee the Pots- 

 dam Group is divided into three great sub-groups, in ascending order as follows: 

 1. The Ocoee conglomerate and slates; 2. Chilhowee sandstone; 3. Knox 

 Group. The Ocoee conglomerate and slates takes its name from Ocoee river, 

 and is 10,000 feet thick. The Chilhowee sandstone takes its name from the 

 Chilhowee Mountain, and is 2,000 feet thick. The Knox Group takes its name 

 from Knox county, and is 7,000 feet thick; making the total thickness of the 

 Potsdam in Tennessee 19,000 feet. It is from 500 to 2,500 feet thick in Canada, 

 2 020 at Bonne Bay, Newfoundland, 1,147 feet at the Straits of Belle Isle, about ■ 

 2,000 feet in Pennsylvania and West Virginia, and from 600 to 800 feet thick in 

 Wisconsin and Minnesota. 



