90 A. WINCHELL^A LAST WORD WITH THE HUEONIAN. 



Murray and Logan on the Region South of the Ottawa. — In 1851 Director 

 Logan and Assistant Murray examined different portions of the district be- 

 tween the Ottawa and the St. Lawrence. As this region may well be re- 

 garded as an extension of the geological district north of Lake Huron, some 

 passages from this report may be cited. 



Mr. Logan describes the Potsdam sandstone as reposing unconformably 

 on the " Metamorphic or gneissoid group " (p. 6). This group, as thus 

 understood, embraced the body of rocks extending from the Potsdam sand- 

 stone to the granites and gneisses. Of the character of this group in the 

 seigniory of Rigaud, Mr. Murray, after speaking of the gneisses, says : * 



" These beds are interstratified with others of a different character. One set is com- 

 posed of small cleavable forms of black hornblende and grains of translucent, yel- 

 lowish-white feldspar, weathering opaque-white, and crystals of brown mica [mica- 

 ceous, quartzless syenite (hyposyenite)] ; another consists of grayish-green, cleavable 

 pyroxene, with individuals of greenish feldspar, weathering white, and largely dis- 

 seminated grains of magnetic iron [gabbro], and a third consists of translucent 

 albite, with black hornblende and magnetic iron ore disseminated [rnagnetitic dio- 

 rite] alternating with micaceous layers. All these beds are intersected by transverse 

 dikes, some of which are fine-grained, grayish-black trap, probably a greenstone, with 

 disseminated grains of calc-spar, while others are porphyritic, having a fine-grained, 

 blackish-green base, with individuals of greenish- white feldspar." 



It is difficult to form a clear conception of a formation thus constituted. 

 Though it rests, like the lowest strata along Thessalon and Spanish rivers, 

 immediately above the gneisses, the description of it sounds extremely unlike 

 the descriptions cited of the Thessalon and Spanish river strata. 



Logan on Rocks North of Lake Huron. — Director Logan again, in 1852, 

 returned to a notice of the rocks north of Lake Huron.f He says : 



" On Lake Huron the Lower Silurian group rests unconformably upon a silicious 

 series, with only one known band of limestone, 150 feet thick, with leaves of chert in 

 abundance, but as yet without dicovered fossils. This series is supposed to be of the 

 Cambrian epoch. It comprehends the copper-bearing rocks of that district, and with 

 its igneous, interstratified masses has a thickness of at least 10,000 feet." 



Introduction of the Name " Laurentian." — In 1854 the term "Laurentian" 

 was introduced by Logan in the following words : J 



" The name which has been given in previous reports to the rocks underlying the 

 fossiliferous limestones in this part of Canada is " the Metamorphic series; " but in- 

 asmuch as this is applicable to any series of rocks in an altered condition and might 

 occasion confusion, it has been considered expedient to apply to them for the future 

 the more distinctive appellation of the Laiirentian series^ a name founded on that 

 given by Mr. Garneau to the chain of hills which they compose." 



This term, therefore, was at first employed for the entire series of rocks 



* Report of Progress for 1851, p. 63. 



t Quart. Journ. Greolog. Soc, vol. VIII, 1852, p. 200. 



i Report of Progress, Geol. Surv. of Canada, 1852-'53, Quebec, 1854, p. 8. 



