100 The Position of the Cincinnati Group. 



gan (Geo. of Can. 1866, p. 19) describes the Grenville band in which 

 the fossil is found as follows : 



"The general character of the rock connected with the fossil pro- 

 duces the impression that it is a great foraminiferal reef, in which the 

 pyroxene masses represent a more ancient portion, which having died, 

 and become much broken up, and worn into cavities and deep recesses, 

 afforded a seat for a new growth of forarainifera, represented by the cal- 

 careo serpentinous part. This in its turn became broken up, leaving, 

 however, in some places, uninjured portions of the organic structure. 

 The main difference between this foraminiferal reef, and more recent 

 coral reefs, seems to be, that while with the latter are usually asso- 

 ciated many shells and other organic remains, in the more ancient one 

 the only remains yet found are those of the animal which built the 

 reef." 



The next series of rocks overlying the Laurentian are called the 

 Huronian, which, on the north shore of Lake Huron and to the east- 

 ward, consists of quartzites, chloritic slates, bands of limestone chert, 

 jasper and slate conglomerates, not less than 18,000 feet in thickness. 



On Lake Superior, Sault Ste. Marie, Mamainse, and other places, they 

 are exposed from 10,000 to 16,208 feet in thickness. An approxi- 

 mate estimate of the thickness of this series on Michipicoten Island, 

 says Mr. MacFarlane, is 18,500 feet. And if we compare the rocks of 

 Michipicoten Island with those of Mamainse, it would appear that the 

 lower rocks of the latter series do not come to the surface at Mich- 

 ipicoten Island, and that the higher rocks of the Michipicoten se- 

 ries have not been developed at Mamainse, or lie beneath the waters 

 of the lake to the southwest of the promontory. It would there- 

 fore appear just in estimating the thickness of the upper copper- 

 bearing rocks of the eastei'n part of Lake Superior (which are Huron- 

 ian), to add to the Mamainse series the above mentioned 4,000 feet 

 of resinous traps, or porphyrites, which would make the whole thick- 

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

 of Can., 1866, pp. 182, 141.) 



The metamorphic strata, equivalent to the Laurentian and Huronian 

 series of Canada, is described in Safford's Geological Survey of Ten- 

 nessee, p. 178, as many thousand feet thick in that State. Fossil re- 

 mains of foraminifera have been found in it. They have also been 

 found in the metamorphic rocks of Europe, several thousand feet be- 

 low their surface. 



Next above the Huronian or metamorphic rocks lies the great Silu- 

 rian formation, named by Sir K. I. Murchison, in memory of the an- 

 cient Silures who inliabited Wales, where he first studied the exposure 



