I 



SIR WILLI A>[ i:i»M(»ND LOGAN". 359 



linu'stones," which were said to be disliiictly iiiterlietldtMl. Kesiing 

 upon tliese on Lake Tomiscaining, Loi^au described in the same 

 report a newer series, cliieHy of eldorifie shites holdinjf pebbk^s of the 

 underlying gneiss; and in his report of his examination of Lake Supe- 

 rior in 1(S45 (also published in 1847), these two seiies were distinctly 

 indicated as a lower formation of granitic gneiss, often syenitic, and an 

 ui)per one of micaceous, chloritic. and talcose slates, freijnently with 

 epidote, associated with hornblendic rocks and greenstones, (piartzites, 

 and conglomerates including pebbles of the older rocks ; this up[)er 

 series being probably several thonsand feet in thickness. 



Li their report for 1851 on the geology of Lake Superior, Messrs. 

 Foster and Whitney also described these crystalline rocks, including 

 the two divisions, as the Azoic system, which they recognized as of 

 sedimentary origin. The fiirther studies of the Canadian survey 

 established the importance of the two divisions, and the necessity of 

 separate designations for them; and in Logan's report for 1853 (pub- 

 lished in 1854) the name of the Laurentian series was given to the 

 lower formation, which forms the chief part of the elevated region to 

 the north-west of the St. Lawrence to which the title of the Laurentide 

 Mountains had been previously assigned. The name of Laurentian has 

 since been adopted for the similar rocks of Continental Europe and 

 of the British Isles. In 1855, the designation of Huronian was given 

 by the Canadian survey to the upper division, including the series 

 characterized by greenstones and talcose and chloritic schists which is 

 largely developed on the sliores of Lakes Huron and Superior (where 

 it had been carefully studied and mapped by Mr. Alexander Mui-ray), 

 and constitutes tlie Huron Mountains to the south of the latter lake. 



The subsequent labors of Logan on the Ottawa established clearly 

 the regularly stratified character of the Laurentian series, of which he 

 measured about 20,000 feet, consisting of four gneiss formations sepa- 

 rated by three limestones, each of the latter having a thickness of 

 from 1,000 to 1,500 feet, and associated with quartzites ; the whole con- 

 stituting a series comparable in value to the entire lower Paleozoic. 

 These strata, greatly affected by undulations and penetrated by eruptive 

 rocks, were by Logan traced with infinite labor over an area of 2,000 

 square miles ; and a geological map of this region, published by him in 

 the Atlas to the Geology of Canada in 1863. is the first attempt to un- 

 ravel the stratigraphy of this most ancient and disturbed series of rocks. 



At the summit of this series. was found a mass of about 10,000 feet 

 of stratified crystalline rocks, which, unlike those below, consisted 

 chiefly of labradorite and hypersthene rocks, with some little included 



