'54 



NATURE 



[June i6, 1892 



much questioning.^ The work is easy in its methods, yet 

 perplexing because in North America the uplifts and 

 flexures of different periods have in general taken place 

 in parallel directions, so that unconformabilities are dis- 

 guised, especially when the two formations are nearly 

 alike in grade of metamorphism. Follow along the over- 

 lying to places where its metamorphism is of low grade, 

 and there may be success. 



There is a first point of special importance to be accom- 

 plished by Archaean investigation. The Huronian of the 

 Penokee-Marquette region is partially metamorphic. To 

 the east, the iron ore, according to the describers, is 

 mainly metamorphic magnetite and hematite ; to the 

 west, especially in the Penokee region, it is largely iron 

 carbonate, or the ore in its original state. Other facts 

 show a diminishing grade of metamorphism to the west- 

 ward. In the Penokee district, the ore is underlain by a 

 bed of " cherty limestone," the chert of which, like the 

 interlaminated jasper of the iron ore bed, is regarded by 

 Van Hise as probably of organic origin, like later chert. 

 It has among the overlying beds carbonaceous shales 

 containing, according to Chamberlin, 40 per cent, of 

 carbon, bearing thus evidence of very large organic car- 

 bonaceous contributions when in process of formation. 

 The great beds of iron ore, the upward gradation east- 

 ward in metamorphism, the relations in position to the 

 admitted Archaean adjoining it on the south, seem to 

 prove the Huronian series to be Upper Archaean, as it 

 has been generally regarded, but in a non-metamorphic 

 and partially metamorphic condition. The question 

 thence arises : Are the ore-bearing rocks of the Archaean 

 of Eastern Canada, New York, New Jersey, and other 

 parts of the Appalachian chain, Huronian in a state of 

 high-grade metamorphism ? Are the chondroditic lime- 

 stones, which, in some localities, occur in and with the 

 ore, part of the Huronian formation } Does the eastern 

 iron-bearing series rest unconformably on inferior 

 Archaean ? 



The Algonkiati (or Agnotozoic) beds belong either to 

 the Archaean or to the Paleozoic. 



The ArchiEan division of geological time is of the same 

 category with the Paleozoic, Mesozoic,and Cenozoic ; all 

 are grand divisions based on the progress of life, and they 

 include together its complete range. There is no room 

 for another grand division between Archaean and 

 Paleozoic atiy more than for one between Paleozoic and 

 Mesozoic, In contrast, the Algonkian division is not 

 above the Cambrian in grade, it being based on series of 

 rocks. Its true biological relations are in doubt, because 

 fossils representing the supposed life of the period are 

 unknown, or imperfectly so. The discovery in any rock 

 so-called of Trilobites, Crustaceans, Mollusks, Brachio- 

 pods, or Crinoids, whatever the species, would entitle 

 such rocks to a place in the Paleozoic, and either within 

 the Cambrian group or below it. Walcott has already 

 reported such fossils from the beds at the bottom of the 

 Colorado cafion referred by him to the Algonkian— namely, 



I As a preliminary in the study of any such region, thousands of dips and 

 strikes of planes of foliation or bedding should be taken (in imitati n of 

 Percival's work before 1842, mentioned in the note on p. 440 of the last 

 volume of the American Journal of Science), and all should be plotted on 

 maps of large scale by means of symbols with affixed numbers recording the 

 dips and strikes, for full comparison in the final elaboration. Even the 

 Penokee-Marquette region needs further investigation with a clinometer- 

 compass in hand , , r n 



Before commencing the study of any crystalline rocks, models of Hexures 

 should have been studied until the fact is fully appreciated that a flexure 

 having an inclined axis— the commonest kind— ranges through 180 , or 

 nearly, in its dips and strikes, and until the characters of the bedding in 

 different transverse sections of flexures are well apprehended. A good 

 model for studying flexures may be made from a cylindrical stick of coarse- 

 grained wood having the bark on (if of a smooth kind) ; it may be about four 

 .nches in diameter and twelve to fifteen long. Draw a straight line through 

 the centre of one end ; and from this line saw acro'^s obliquely to the edge at 

 the opposite end. After planing smooth the sawed surface, the layers of the 

 wood may then be coloured by groups ; and three colours, or two besides 

 that of the wood, are better than more. The model of a flexure having aii 

 inclined axis is then complete. Cross-sections of the model may be cut and 

 the colours added to the new surfaces. For models of overthrust flexures, 

 this method is not practicable, as wood of elliptical section would be re- 

 quired. They may be made of paper-pulp of three colours. 



besides a Stromatoporid, a small Patella-like or Discina- 

 like shell, a fragment of a Trilobite and a small Hyolithes 

 — forms which make the beds Paleozoic beyond question. 



3. Subdivisions based on Physical and Biological 

 Conditions. 

 Although the physical and biological conditions of the 

 early globe are within the range of observation, there are 

 generally admitted facts which afford a basis for a philo- 

 sophical division of the time ; and from it geology may 

 derive instruction. The subdivisions to which we are led 

 are the following : — 



I. The ASTR.'VL aeon, as it has been called, or that of 

 liquidity. 



II. The Azoic aeon, or that without life. 



(i) The Lithic era, commencing with completed 

 consolidation : the time when lateral pressure for crust- 

 disturbance and mountain-making was initiated, and when 

 metamorphic work began. 



(2) The Oceanic era, commencing with the ocean in 

 its place : oceanic waves and currents and embryo rivers 

 beginning their work about emerged and emerging lands, 

 and the tides, the retarding of the earth's rotation. 



III. The Archeozoic aeon, or that of the first life. 



(i) The era of the first Plants : the Algae and later the 

 aquatic Fungi (Bacteria) ; commencing possibly with the 

 mean surface temperature of the ocean about 1 80° F. 



(2) The era of the first Animal life : the Protozoans, 

 and forms related to the embryos of higher invertebrate 

 species ; commencing possibly with the mean surface 

 temperature of the waters about 120^ F., and ending with 

 90° F. or below. 



The subdivisions, as is evident, mark off great steps in 

 the progress of the developing earth, although the rocks 

 bear no marks of them that can be distinguished. 



The Huronian period covered, probably, much of 

 Archasozoic time ; and this is all in the way of correlation 

 that can be said. It is well to note here that if the 

 Eozoon is really animal in origin, the" Laurentian" rocks 

 of Canada in which it occurs must be Huronian, or the 

 later of Archaean terranes. 



Respecting the Oceanic period it is observed above, 

 " cotnmencing with the ocean in its place." It appears to 

 be almost a physical necessity that the oceanic depres- 

 sion should have been made in the first forming of the 

 solid crust, if the globe cooled to the surface from the 

 centre outward ; that is, unless a liquid layer remained 

 long afterward beneath the crust. 



The depression was certainly made long before the 

 close of Archaean time. For the enormous amount of 

 rock-making of the Archaean over the continent imphes 

 the existence of emerged rocks with reach of the decom- 

 posing, eroding, and denuding agencies of the atmosphere 

 and atmospheric and oceanic waters. A submergence 

 in the ocean of 50 feet is almost a complete protection 

 against mechanical and chemical wear. Moreover North 

 America has its Archaean lands not only m the great 

 nucleal mass, 2,000,000 square miles m area, but also m 

 the series of Archaean ranges parallel to the outlmes of 

 the nucleus, which extend eastward to the eastern limit 

 of Newfoundland, and westward to the Pacific. And it 

 has correspondingly shallow-water Cambrian deposits 

 lying between these ranges from Eastern Newfoundland 

 and the coast-region of New Burnswick and Massa- 

 chusetts, westward across the continentabout most of the 

 Archeean outcrops, to within 300 to 400 miles of the Pacific 

 Ocean, as shown by Walcott. , . . .1, 



There is hence reason for the conclusion that, at the 

 close of Archaean time, the continent of North America 

 was present not merely in outhne, but also in general 

 features, and at shallow depths where not emerged. 



This fact with reference to North America means much. 

 It means that by the end of Archaean time, the continents 

 generally were essentially in a like condition— outlined 



NO. I 181, VOL. 46] 



