SCIENCE 



NEW YORK, MAY 36, 1893. 



THE NOEIAN ROCKS OF CANADA. 



BY ANDREW C. LAWSON, GEOLOGICAL LABOKATOKY, tlNlVERSITY OF 

 CALIFORNIA, BERKELEY, CAL. 



An important advance has been made in American Archaean 

 geology by the publication of a memoir, by Professor Franlj D. 

 Adams of McGill College, Montreal, on the Norian or Upper Lau- 

 rentian rocks of Canada.' The memoir embraces the results of 

 five seasons' field-work conducted for the Geological Survey of 

 Canada, together with an exhaustive and masterly petrographical 

 investigation in the laboratory of the rocks constituting the Norian 

 terranes. A full bibliography of these interesting rocks accom- 

 panies the memoir; also a map showing their knovsri distribution, 

 and a table of twenty-four analyses compiled from various 

 sources. Thoroughness of research and soundness of judgment 

 characterize the work throughout, and all interested in Archaean 

 geology or in general petrography will welcome it as one of the 

 sure steps which make for scientific progress. The author intro- 

 duces his subject by a general presentation of the geology of the 

 Laurentian, in which he gives a brief history of the growth of 

 our knowledge and theories of these rocks, and discusses their 

 nomenclature, both geological and petrographical. In the latter 

 connection he takes occasion to describe, in general terms, the rock 

 of which the Norian is prevalently constituted, and makes good 

 its claim to be regai'ded as a distinct petrographical type, the 

 characteristic of wliich is the preponderance of plagioclase to the 

 partial or entire exclusion of the ferro magnesian silicates. For 

 this type of rock he uses the term ''anorthosite," in accordance 

 with a usage which has long been in vogue among the Canadian 

 geologists. The anorthosites are regarded as belonging to the 

 gabbro family, constituting one extreme of a graded series, of 

 which pyroxmites and other granular rocks, rich in ferro- mag- 

 nesian minerals to the practical exclusion of feldspar, are the 

 other extreme, while common gabbros, norites, etc., are the mid- 

 dle members. 



The various regions occupied by these anorthosites are described, 

 and the petrographical and geological relations are discussed in 

 detail. Their occurrence appears to have a peculiar relation to 

 the so-called Archasan nucleus of the continent. So far as ex- 

 plorations serve us, they are only known to occur on the periphery 

 of this great "Canadian shield," along the line of the St. Law- 

 rence drainage, from near its source to the Straits of Belle Isle, 

 and again farther north on the LabiM..,"-.- c3l': ;. !:'":'essor Adams 

 calls attention to the seeming analogy between this distribution 

 and the modem distribution of volcanoes on the periphery of the 

 continents. The more important anorthosite occurrences, both 

 by reason of their great extent and for the greater study that has 

 been given to them, are (1) the Morin region, north of Montreal, 

 and (3) the Saguenay region, which is the largest known. The 

 Morin mass has a diameter of 37 miles and an area of 990 square 

 miles. It is surrounded by the gneisses, crystalline limestones, 

 quartzites, etc., of the Grenville series (Laurentian), through 

 which it is clearly eruptive. It holds included in it blocks of the 

 surrounding gneiss, and the latter is traversed by apophyses 

 from the main mass; and at places distinct evidence of an altera- 

 tion contact-zone is observable. The deeply-worn surface of 

 both the Laurentian and the invading anorthosite is overlaid by 

 flat- lying, unaltered Cambrian strata. The age of the anorthosite 

 is thus limited as Post-Laurentian and Pre-Cambfian. Petro- 

 graphically, the anorthosites are characterized by a peculiar cata 



■ TJeber das Norian Oder Ober-Laureutlan von Canada. Stuttgart, 189-3. 



clastic structure, which is most pronounced where the rock mass 

 evinces a schistose or foliated structure. This cataclastic structure 

 is ascribed to pressure acting on the mass and deforming it while 

 it was yet deep in the crust of the earth, and at a temperature 

 probably near its fusion point. In addition to the dominant 

 mineral plasioclase, many other rock-forming minerals occur, 

 such as augite. hypasthene, ilmenite, hornblende, biotite, etc. 

 Most of these play a very subordinate role as normal constituents, 

 while others are rare accessories; and some are secondary or de- 

 composition products. Certain layers of anorthosite occur inter- 

 calated with the gneiss and crystalline limestone of the surround- 

 ing Grenville series. These layers of apparently interbedded 

 anorthosites vary in thickness from one yard to several hundred 

 yards, and in length from half a mile to eight miles. The ap- 

 parent interbedding is due to the intrusion of the anorthosite 

 within the strata of the Grenville series. 



The Saguenay region presents a mass of anorthosite of still 

 more extensive proportions, occupying not less than 5,800 square 

 miles. The anorthosite of this region is essentially similar to 

 that of the Morin region, both petrographically and in its relations 

 to the Laurentian and to the base of the Palseozoio. There are, 

 however, some differences in petrographical detail ; one of these 

 being the common occurrence of olivine among the ferro-mag- 

 nesian constituents. The Laurentian gneiss of the surrounding 

 region corresponds rather to the Ottawa gneiss than to the Gren- 

 ville series, and the contact phenomena on the periphery of the 

 anorthosite mass are more confused than in the case of the Morin 

 mass. 



The other localities where occurrences of anorthosite rocks 

 are described are: In Labrador, in Newfoundland, on the north 

 side of the St. Lawrence River and Gulf, in the State of New 

 York, and on the east coast of Lake Huron. To this list of locali- 

 ties the present writer may be permitted to add northern New 

 Jersey and the northwest coast of Lake Superior, where he has 

 met with anorthosite rocks in the field. 



Among the more important results of Professor Adams's work 

 may be mentioned: — 



1. The clear recognition, as plutonic eruptive formations, of 

 rock masses, which, being petrographically and geologically units, 

 have each an enormous extent. The Morin mass occupies nearly 

 1,000 square miles, and the Saguenay mass is at least about six 

 times that area. For such irruptive plutonic masses Suess has 

 proposed the designation "batholite," and it will facilitate dis- 

 cussion to use that term. The recognition and description of the 

 laccolitic type of mountain structure by Gilbert has been prolific 

 of very fruitful results. The fact that there are such structures was 

 so ably presented by their discoverer that it was warmly and gen- 

 erally received, and it has become an important, element in dis- 

 cussions of orogeny. The fact that there are batbolites has not 

 had so vigorous a presentation, and the important role which they 

 play in the structure of the earth's crust has not received the 

 recognition which its importance merits. The able account which 

 Professor Adams gives us of these great batbolites of anorthosite 

 is, therefore, a welcome addition to our knowledge of such struc- 

 tures. The present writer has elsewhere endeavored to call 

 attention to the existence of granitic batbolites in the Rainy Lake 

 region, which are comparable in size to the anorthosite areas of 

 Quebec; and it seems to him that a more general appreciation of 

 the importance of these great Igneous masses would simplify and 

 advance our discussion of certain problems of tectonic geology, 

 metamorphism, and geognosy. If it be a fact, as Professor 

 Adams and the writer have shown for two distinct regions, that 

 immense masses of igneous matter, ranging from 35 to 100 miles 

 or more in diameter, have invaded the crust from below, dis- 



