OEOLOaiCAL IS'OTES IN THE GREAT EXIIiniTION. 



385 



A considerable portion of tlic rocks of the Quebec group in tlicir mota- 

 morpln'c condition consists of cliloritie slates, AAliich np])('ar to occupy a 

 liii>lier strati<^rapliical position than the inaLjiicsiaii sli'ala just noticed. 

 Mai^nesian mica, or phlo^o])ite, occurs al)uiulaut]y in small scales in the 

 crystalline limestone of the Laurentian system, hut sometimes also in 

 crystals sufficiently lar<>e to be economically available. Among its asso- 

 ciated minerals are connnonly quartz, pyroxene, and fcMspai', and occa- 

 sionally tabular spar, apatite, spheric iron pyrites, idoci asc, garnet, tour- 

 maline, zircon, and corundum. In Grcnville crystals of mica have been 

 obtained, giving sheets measuring 24 by 14 inches. In North Burgess, 

 where it has been mined, the mica is embedded in a soft pyroxenic rock 

 and iijaiited by a band of quartzite on the south side. The mica there ap- 

 pears to run for 75 yards in tolerably regular bands, and some of the 

 sheets after being dressed are as much as 20 inches S(]uare ; some have 

 been obtained measuring 20 by 30 inches. The crystalline limestones of 

 the Laurentian system are marked almost universally by the occurrence 

 of gra])hite or plumbago in small scales, which are often so thickly disse- 

 minated in particular bands of the rock as to give them a black or dark 

 grey colour, distinctly marking the stratification. Plumbago also occurs 

 in beds, of sufficient purity and quality to be economically available. The 

 workable beds are chiefly on the north side of the Ottawa, and occur in 

 many localities at considerable distances from one another, but several of 

 the exposures are probably rc]:)etitions of the same bed, or at any rate of 

 beds approximately equivalent in repetition of the same band of Laurentian 

 limestone. The whole Laurentian series is so corrugated that the outcrop 

 of one of these bands of limestone in the counties of Argenteuil and Two 

 Mountains, followed through all its windings in an area of fifty miles by 

 tvvent}', measures upwards of two hundred miles. Abed of pure graphite 

 occurs in the augmentation of Grenville township, and has been traced at 

 intervals for a distance of three miles. One of those exposures has been 

 mined by Messrs. lUissell & Co., and at the opening of the excavation the 

 graphite showed a tliickness of ten inches ; but the pure substance was 

 found to form a lenticular mass, separated from other masses of the same 

 character by intervals in which the graphite became mixed with the lime- 

 stone. It is probable that a number of these lenticular masses running 

 through the rock at the same horizon may represent the general character 

 of the workable beds. Asbestos, generally a fibrous serpentine or clnyso- 

 lite, occurs in veins cutting the serpentine of the Quebec group in the 

 Eastern Townships (St. Joseph Seigniory). A friable sandstone in the 

 Potsdam formation occurs at Pittsburgh, twenty feet thick, and is much 

 I in demand for ])rotecting the sides and bottoms of iron foundries. It is sup- 

 I plied to those at Montreal and Toronto, at distances of a hundred and se- 

 venty miles in opposite directions. 



Of the min(>rais applicable to common or decorative construction it 

 would be perhaps of no great service to English geologists to give very 

 minute details. Still however the characters of the sanipk\s, as indicative 

 of the nature of the rocks constituting the formations to which they belong, 

 will be at least interesting. "We begin with a sample from one of the bands 

 of crystalline linu'stone of the Laurentian series from the Lac des Chats. 

 Another building-stone comes from Phillipsburg (St. Armand) ; the rock 

 is compact and crystalline, and of considerable strength. A few obscure 

 fossils show the formation to be the Caleiferous of the Quebec group. 

 There are s])eeimens of building-stones from Caughnawagu. St. l>omiui(pu% 

 and East Ilawkesbury, all fnnn the Chazy formation, wliich in those tlis- 

 tricts is composed of massive beds yielding blocks of stone tit ted for canal 

 VOL. Y. ' :] J) 



