18 LAURENTIAN SYSTEM. 



from Syene, in Egypt) ; and if it only partially displaces it, it is syenitic granite. 

 Many of the granites and syenites are intrusive, while others, not distinguishable from 

 these, take the place of sedimentation and pass into gneiss or mica schist. Felspar 

 signifies rock-spar from the German word fels, a rock, though it is usually spelled 

 feldspar from the German word feld, a field, and therefore made to signify field-spar. 

 There are several species of felspar, dependent upon the potash, soda, or lime they 

 contain. That which usually enters granite is ortlioclase, or potash felspar, and is 

 compact laminated, or compact crypto-crystalline, consisting- of about the following 

 substances: silica 64.6, alumina 18.5, and potash 16.9. When soda enters into the 

 composition of the felspar, it becomes aUrite, and the granite is then disposed to 

 undergo spontaneous disintegration, which sometimes takes place below direct atmos- 

 pheric influences at great depths in the earth. The kaolin of the Chinese is derived 

 from felspar from the disintegration of granitic rocks, and porcelain clay is often from 

 the same source. Garnets are common in gneiss and mica schist. The most common 

 mica, and that which generally enters into granite, gneiss, and related rocks, is called 

 muscovite. Other species in the mica group are called phlogopite, biotite, lepidome- 

 lane, astrophyllite, lepidolite, and cryophyllite. 



§ 31. Logan said of the Laurentian System: "Stretching on the north side of 

 the St. Lawrence from Labrador to Lake Huron, this series occupies by far the 

 larger portion of Canada, and its strata probably possess a great thickness. To 

 determine the superposition of the various members of such an ancient series of rocks 

 is a task which has never yet been accomplished in geology, and the difficulties 

 attending it arise from the absence of fossils to characterize its different members. 

 Bands of the crystalline limestone are easily distinguished from bands of the gneiss ; 

 but it is scarcely possible to know from local inspection whether any mass of limestone 

 in one part is equivalent to a certain mass in another. They all resemble one another 

 lithologically, and although masses dipping in the same direction are met with, 

 running for considerable distances rudely parellel with one another, it is scarcely 

 ever safe to take for granted that they are stratigraphically distinct. The dips avail 

 but little in tracing out the structure ; for in the numerous folds of the series the 

 dips are frequently overturned, and the only reliable mode of pursuing the 

 investigation and working out the physical structure, is patiently and continuously to 

 follow the outcrop of each important mass in all its windings as far as it can be 

 traced, until it becomes covered up by superior, unconformable strata; is cut off by 

 a great dislocation, or disappears by thinning out." 



§ 32. The surface area accupied by the Laurentian series in Canada and British 

 America, exclusive of any exposure that may exist in the Cordillera or Rocky 

 Mountains, is not less than 250,000 square miles. The northern limit is the Arctic 

 Ocean ; from here it may be traced south upon the western side of Hudson's Bay, 

 and appearing upon its eastern side it spreads over the greater part of Labrador, and 

 extends to the Gulf of St. Lawrence. The southern limit is the St. Lawrence from 

 Labrador to Cape Tourmente, a distance of 600 miles, except a narrow border of 

 Taconic on the Strait of Belle Isle ; another at the mouth of the Mingan River ; a 

 third near the Seven Islands, and two on Murray Bay River, and the Gouffre. Ex- 

 tending westwardly it occurs 30 miles north of Montreal, and follows up the Ottawa 

 River for a distance. It then strikes off to the Thousand Islands, and crosses over, 

 into New York, where it exposes an area of 10,000 square miles. From there it 



