August 26. 1897] 



NA TURE 



399 



granitic and gneissic rocks, forming conglomerates, which may 

 often be observed to pass into schists, but still plainly indicate 

 that, in these places at least, materials not unlike those of the 

 Fundamental Gneiss and its associates were at the surface and 

 subject to denudation. Such materials cannot be regarded as 

 parts of any primeval superficial crust of the earth in an 

 original condition. They represent crystalline rocks formed at 

 great depths, and under conditious similar, at least, to those under 

 which the Fundamental Gneiss was produced. They imply a 

 great pre-Huronian denudation, and show that the Huionian 

 must have been deposited unconformably either upon the 

 Fundamental Gneiss itself, or upon rocks occupying its position 

 and very similar to it in character. There can be no reasonable 

 doubt that the mass of what now constitutes the Fundamental 

 Gneiss originally existed as the floor upon which the Huronian 

 was deposited. 



The name Archrean has been adopted and employed by the 

 Geological Survey of Canada in the sense in which it was intro- 

 duced (in 1874), and consistently maintained by Dana — i.e. to 

 include all rocks below the great hiatus of which evidence was 

 first found in the Lake Superior region. The author of the 

 name never assented to its restricted application as proposed by 

 Irving and followed by Van Hise and others, and as a synonym 

 for the Fundamental Gneiss or " Basement Complex " it is not 

 only unnecessary but is scarcely etymologically correct, if we 

 admit that a part of the " Complex " is of comparatively late 

 date. 



We have reached a point at which we may ask what is now 

 our conception of these Archrean rocks in Canada, and more 

 particularly in the great Protaxis, as resulting from the most 

 recent investigations of a critical kind. The reply may be given 

 briefly from the latest reports of those still at work on the 

 problems involved as follows : — 



The Latiretttian comprises (i) the Fundamental Gneiss or 

 Lower Laurentian (also referred to as the Ottawa Gneiss or 

 Trembling Mountain Gneiss in older Reports), and (2) the Gren- 

 ville Series. An important part of the gneisses of the Grenville 

 Series has been shown by chemical analysis to be identical in 

 composition with ordinary Paleozoic argillites, and they are inter- 

 bedded with quartzites and massive limestones, also evidently 

 of aqueous origin, and in some places abounding in graphite. 

 These beds are, however, closely associated with other gneisses 

 in which orthoclase largely preponderates that have the composi- 

 tion of igneous rocks. The Fundamental Gneiss consists chiefly, 

 if not exclusively, of rocks of the last-named class, the banding 

 of foliation of which, though now generally parallel to that of 

 the Grenville Series, has probably been produced mainly or 

 entirely by movements induced by pressure, in a mass originally 

 differing more or less in composition in its different parts. These 

 two series are sometimes separable on the ground locally, but 

 with difficulty ; in other places they cannot be clearly defined 

 ((/. Adams, Annual Report Geological Survey of Canada, 1895). 

 The Upper Laurentian, Labradorian, Norian or Anorthosite 

 group, maintained for a number of years on the evidence 

 already mentioned, is found to consist essentially of intrusive 

 rocks, often foliated by pressure, later in age than the Grenville 

 Series, but in all probability pre-Palreozoic. 



The Htirotiian comprises felspathic sandstone or greywacke 

 more or less tufaceous in origin, quartzites and arkoses passing 

 into quartzose conglomerates and breccia conglomerates, often 

 with large fragments of many different varieties of granite, 

 syenite, &r., diorite, diabase, limestones, and shales or slates 

 changing to phyllites in contact with the numerous associated 

 igneous masses. Over wide areas altered greenstones and their 

 associated tuff's preponderate, often with micaceous, chloritic. 

 sericitic and other schists, many of which are of pyroclastic 

 origin, although some may represent ordinary aqueous deposits, 

 and all have been much affiected by subsequent dynamic meta- 

 morphism. 



The Huronian rocks have not yet been found in distinct rela- 

 tion to those of the Grenville Series, but are generally in con- 

 tact with the Fundamental Gneiss, in the manner previously 

 alluded to. Where not composed of volcanic material it appears 

 to be largely of a littoral character, while the Grenville Series 

 seems rather to indicate oceanic conditions. 



No reference has so far been made to the development of 

 Archrean rocks, known as the "Hastings series." The rocks 

 thus named occupy considerable tracts to the south of the 

 Ottawa River, west of the City of Ottawa. They were 

 originally classed by Logan and Murray with the Grenville 



NO. 1452, VOL. 56] 



Series of the Laurentian, although Murray soon after insisted onr 

 their peculiar features, and they came to be recognised by the 

 above geographical name during subsequent discussions as to 

 their systematic position, by the authors above referred to, an<P 

 by Hunt, Vennor, and Macfarlane. These rocks are particularly 

 alluded to now, because later work seems to show that both the 

 Grenville Series and the Huronian are represented in the district 

 — in so far, at least, as lithological characters may be depended! 

 on. They include a preponderance of thinly-bedded limestones 

 and dolomites, finer in grain and usually less altered than those 

 of the typical Grenville Series, associated with conglomerates., 

 breccias and slates still retaining complete evidence of their 

 clastic origin. 



It is in this Hastings region that careful investigation and) 

 mapping are now in progress by several members of the Canadian- 

 Survey, with the prospect of arriving at definite results respect- 

 ing the relations of the Grenville Series and the Huronian. It 

 is too early to forecast what these results may be, for the- 

 question is one which must be approached with an open mind ;: 

 but the work already completed by Messrs. Adams, Barlow, and 

 Ells, appears to sustain the suggestion that both series occur,, 

 and to indicate that they may there be so intimately connected as^ 

 to render their separation difficult. It must be borne in mind 

 that, although the relations of the Grenville Series and those of 

 the recognised Huronian to the Fundamental Gneiss are very- 

 similar, they characterise distinct tracts, to which the Hastings- 

 district is to some extent geographically intermediate, although 

 most closely connected in this respect with the Grenville region. 

 Reverting to the original classification of the Archaean of the 

 Canadian Survey, as developed in the field by Logan and his- 

 assistants, we may now inquire — In how far does this agree 

 with the results of later work above outlined ? In the main,, 

 this classification still stands substantially unaltered, as the 

 result of all honest work carefully and skilfully executed must^ 

 The nomenclature adopted is still applicable, although some of 

 our conceptions in regard to the rocks included under it have 

 necessarily undergone more or less change. 



The Laurentian is still appropriately made to include both the 

 Fundamental Gneiss and the Grenville Series ; althov^h at first 

 both were supposed to represent " metamorphic " rocks, it was^ 

 even then admitted (1855) that these embraced .some plutonic 

 masses practically inseparable from them. Later investigations- 

 have increased the importance of such plutonic constituents^ 

 while at the same time demonstrating the originally supposed 

 sedimentary origin of the characteristic elements of the Gren- 

 ville Series ; but the admission of so large a plutonic factor- 

 necessarily invalidates in great me.isure the estimates of thickness 

 based upon the older reasoning, under which any parallelism or 

 structure was accepted as evidence of original bedding. 



Whatever views may be held as to the propriety of including 

 rocks of the two classes under a simjle name, the r>ecessity of so» 

 doing remains, because of the practical impossibility of separat- 

 ing them over any considerable area for the purpose of delinea- 

 tion on the map. No advance in knowledge is marked ii> 

 substituting for Laurentian, with its original concept of a strati- 

 fied time-series, such a name as " Basement Complex." It 

 may, indeed, yet prove that the homogeneity of the Laurentian. 

 is greater than is at present supposed, for a mass of strata that, 

 included ordinary sediments, arkoses, and contemporaneous vol- 

 canic deposits of certain kinds, in which thearkose and volcanic- 

 constituents preponderated in the lower beds, might, under 

 metamorphism at great depths, produce just such a con>bination» 

 as that of the Grenville Series and the Fundamental Gneiss, the- 

 latter representing an aggregate result of the alteration of thati 

 part composed chiefly of volcanic material or of arkose — in fact, 

 under the conditions assumed, the lower mass could not now- 

 well exist under any other form than that actually found in the- 

 Fundamental Gneiss. In his address at the Nottingham Meet- 

 ing of this Association, Teall has clearly pointed out that, ii» 

 such cases, the chemical test must necessarily fail, and that the 

 character and association of the rocks themselves must be given- 

 a greater weight. 



The Huronian proper, under whatever local names it may be 

 classed, still remains a readily separable series of rocks, witb 

 peculiar characters, and economically important because of the 

 occurrence in it of valuable minerals. 



The subsequently outlined Labradorian has be€r> eHminated 

 as a member of the time-series, and the rocks of the so-callecB 

 " Hastings Group " remain yet in a doubtful position, but with 

 the promise that they may afford a clue to the true relations of 



