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A careful consideration of all the facts to be observed in the Menominee Region confirms me in this hypothesis,* which is further supported, as it seems to me, by observations in the Penokie Iron Region (Bad River), Wisconsin.

Colonel Whittlesey's maps and sections, given in Owen's Report, 1852, represent a belt of granite, syenite, and hornblende rocks as dividing the Penokie series (Huronian) from the overlying Copper-bearing amygdaloidal traps and sandstones, which lie to the north and nearer the lake.

I observed these rocks at several points in 1871, and noted their general lithological resemblance to the Laurentian, as well as the almost insurmountable structural difficulties in assigning to them that age, and recorded in my notes the probability of their being Upper Huronian. Rowland Irving mentions these rockst as being coarsely crystalline aggregates "chiefly of labradorite and orthoclase feldspar, hornblende, and some variety of pyroxene," with occasional evidences of bedding, which points toward their entire conformability with the underlying Huronian. He regards them as of the period of the Copper-bearing series, constituting its lowest and oldest portion.

Having been, so far as I know, but little studied, it is perhaps impossible at this time to determine their age but what is known can here be briefly surveyed, and an inference drawn, which will not be without value in directing further investiga

tions.

1. The general lithological similarity of this granitoid belt to the Laurentian, has been remarked. It has quite as much similarity, if not more, to several members of the Huronian; and is, I believe, not identical with any rock known to belong to the Copper series.

2. Its geographical extension is peculiar in this: it wedges out rapidly to the east from the vicinity of Penokie Gap, entirely disappearing at the Montreal River, which divides Michigan from Wisconsin. Professor Pumpelly and myself traced the boundary between the Copper and Huronian rocks 30 miles farther eastward beyond Lake Gogebic, without again observing it, which we should certainly have done if it had existed there; for we often found the two series very near together, although the actual contact was not seen.

* Dr. H. Credner regarded the entire Marquette series as the equivalents of the lowest member (a quartzyte) of the Menominee Huronian, a position not at all borne out, as it seems to me, by the facts. He seems to have based this geognostic reasoning largely on a rough section which I sketched for him (and which he has reproduced) of the Negaunee District, where the Upper Huronian, so well developed at Michigamme Lake, is wanting. His great overestimate of the thickness of the Menominee rocks has also led him astray. (See Zeitschrift der deutschen geologischen Gesellschaft, Band xxi, 1867, p. 553.) No attempt was made in my Michigan Report to correlate the Marquette and Menominee series, each being provisionally numbered independently.

+Am. Jour. Sci., vol. viii, 1874, p. 49.

3. Not only does this granitoid formation thin out and disappear in its eastward prolongation, but the same is true of the whole Huronian series, the belt of which becomes narrow as followed east, and finally disappears in the neighborhood of Gogebic, where the Laurentian is seen very near the Copper

series.

*

4. The fact that the granite mass does not cross either the Copper or Huronian series, or, so far as observed, give off dikes in either, renders it improbable that it came into its present position as an eruptive mass subsequent to the formation of both series of rocks.

5. The various ores of iron, which are so generally and abun dantly diffused in the Lower and Middle Huronian, are entirely absent so far as observed from the upper three or four members as developed in the Marquette and Menominee regions, and also in the Penokie series if the following hypothesis is true; but they occur in all forms, although, it is believed, not abundantly, in the uppermost exposed member on Black River. If we suppose this iron to have been mostly precipitated as a carbonate, then we might expect it would be more generally diffused through the rocks of certain epochs than those materials derived from the erosion of adjacent coasts.

There is evidently but one hypothesis which will reconcile these facts, which is: that the granitoid formation in question is of the Huronian period, and probably the youngest member; which series are here non-conformably overlaid by the Copperbearing rocks. I conceive that this view is supported by the observations in the Menominee region above recorded, and suppose this Penokie granitoid formation may be the equivalent of granitic bed XX of the Huronian series as developed in that region. On this hypothesis, it is possible that the valley dividing the Penokie Range proper from the granitoid belt may be underlaid by a soft slate, the equivalent of the micaceous schist, bed XIX.

I would anticipate the objection which many will make to attaching much weight to lithological evidences in determining the age of formations 100 miles apart, by repeating that the staurolitic mica schist formation (XIX) maintains its mineralogical character for over one half this distance. I fail to understand why conditions favorable to the formation of extensive areas of particular rocks may not have existed occasionally in Archæan Time, since they were so prevalent in the folowing ages. This idea of equivalency is further supported by facts given in my "Revised Descriptive Catalogue of the Michigan State Suite of Huronian Rocks," in preparation.

Pumpelly and Brooks, this Jour., vol. iii, 1872.

The best point for observing the Huronian between Lake Gogebic and Montreal River.

The approximate conformability in strike and dip of the Huronian and Copper series, observed by Prof. Pumpelly and myself between the Montreal River and Lake Gogebic,* would, in this view, be only accidental and not prove identity of age, as we were at the time inclined to suppose, and with which view Mr. Irving agreed.

As supporting the view that these pre-Silurian systems † are of distinct periods, I would call attention to their well-known points of difference. The Huronian series of stratified greenstones, chloritic and related schists, clay slates, quartzytes, marbles, micaceous and hornblende schists, gneisses and granites, containing no copper or other metallic ores, except great conformable beds of magnetite, hematite, and limonite, differ as widely as may be from the compact and amygdaloidal melaphyres, friable sandstones, conglomerates with porphyry pebbles, which constitute the bulk of the Copper series, the whole more or less charged with native copper and silver; all of which points strongly toward a different origin for the two systems.

In their metamorphoses and movements subsequent to their deposition, there is a not less wide divergence noticeable. The friable sandstones of the Copper series, showing no greater metamorphism than the overlying Silurian for which they are often mistaken, has no counterpart in the highly crystalline schists and quartzytes of the Huronian, where we have only just enough of the arenaceous character left in some of them, to leave no doubt as to their fragmentary origin. But the difference in the amount, sharpness, and regularity of the folding and bending of the rocks of two systems into existing wave-forms, is if possible wider than their lithological variations. Contrast the magnificent regular sweeps of the Copper series, the main ranges of which preserve the same strike and direction of dip from Keweenaw Point westward for 150 miles, presenting for half the distance only the south upturned edge of the broad synclinal which embraces one fourth of the great lake in its basin; with the older system, everywhere sharply folded into narrow troughs and irregular basins, trending in every direction, the upturned edges of whose enclosing rocks box the compass, winding and zig-zaging in outcrop like a sluggish river.§

* This Jour., vol. iii, June, 1872.

I regard the non-conformability and difference in age of the Copper-bearing series and Lower Silurian rocks of Lake Superior, as established by the facts recorded in the papers of Prof. Pumpelly and myself and of Mr. Irving, in this Journal, already referred to. The hypothesis that the Copper rocks are the youngest Silurian formations of Lake Superior and were deposited during a period of elevation and depression which ceased at the beginning of the St. Mary's (Potsdam) epoch, I conceive is not supported by recently observed facts.

See Irving's interesting remarks, this Jour., vol. viii, July, 1874.

Dr. J. P. Kimball, called attention to this structure in 1865, in this Journal.

If we extend our observations to the older and again nonconformable Laurentian, we find the rocks still more plicated and metamorphosed, often even to the extent of entirely obliterating all evidences of stratification. If we suppose the forces which have produced the metamorphosis and the wave forms to have acted regularly and constantly from the beginning of Archæan time to the beginning of the Paleozoic, we may easily suppose the above results produced, viz.: the Laurentian most disturbed and changed, the Huronian next, and the Copper series least, the Silurian practically not at all.

A fact not without interest is the entire absence, so far as I know, of any patch even of rocks of the Copper period south of the great Keweenaw belt. If the two systems were conformable and of the same age, it is difficult to suppose it possible that erosion should have entirely denuded all the Huronian area which must have been covered by the Copper series of the rocks of that period. One would expect that somewhere a mass of these supposed younger Huronian beds would have been embraced in some one of the numerous sharp, deep synclinals, and have been found by those indefatigable mineral prospectors who have so thoroughly searched this region. On the hypothesis of non-conformability, it is much easier to conceive how it was possible for Silurian breakers coming from the south, slowIv advanced by a subsidence from the same direction, to have done their work in completely uncovering the present Huronian area and leaving the great Copper range escarpment one of the most striking topographical features as well as the most difficult geological problems in the Northwest. It is easy to suppose for example, the horizontal Silurian rocks being entirely eroded from any Archæan terrains, but not of the Huronian rocks being entirely eroded from a Laurentian area, for the reason already given. Lastly, Logan states, Geology of Canada, 1863, p. 77, that "certain conglomerates of the Lower Copper-bearing rocks north of Lake Superior repose non-conformably on the upturned chloritic schists of the Huronian."

We are therefore justified, I think, in regarding the Copperbearing rocks of Lake Superior as a distinct and independent series, marking a definite geological period which separates the Silurian from the Huronian ages. Should future observations confirm this view, it would be advisable to have some more convenient and geologically acceptable name for the series than that now in use. Since Keweenaw Peninsula forms one of the most striking geographical features in Lake Superior and is the locality where the Copper series are best exposed and were first studied, I suggest the name Keweenawian for this period.

*Pumpelly, Credner and myself have observed and recorded this in publications already referred to.

The difference in age of the Huronian and Laurentian having been proven, as already remarked,* by observed non-conformability, by the great rarity, in the younger series, of granite and greenstone dykes so numerous in the older, warrants us in reasserting the same kind and degree of unity and independence regarding the Huronian series.

The considerable amount of carbon distributed through the Huronian, indicating much organic life in that period, leads us to hope that those "imperfect fucoidal impressions seen by Mr. Julien (Mich. Report, vol. ii, p. 5.) may not prove delusive, and that we shall yet be able to avail ourselves of paleontology in determining the age of this system.

The Laurentian rocks have been too little studied to justify an opinion as to whether they may be separated into two or more non-conformable systems, as has been attempted in Canada.

ART. XXIX.-On a new Method of Measuring the Velocity of Electricity; by JOSEPH LOVERING, of Cambridge, Mass.

PERHAPS it is not too strong a statement to say that a question is half answered when it is properly asked. Now when it is asked, What is the velocity of electricity, there is no strict propriety in the question. For electricity has no velocity, in the common sense of the word velocity. There is no analogy between the transmission of an electrical disturbance and the propagation of light, or sound, or radiant heat, for example. The mathematical theory of the galvanic circuit, as stated by Ohm in 1827, and the more recent analysis on the same subject by Kirchhoff and Sir William Thomson, have appeared to prove that the time of transmission of an electrical disturbance is proportional to the total electro-statical capacity of the conductor, multiplied by its total resistance. As each of these factors increases with the length of the conductor, the time of transmission is proportional to the square of the length of the conductor. Therefore, it cannot be told with what velocity electricity will move until it is known through what distance it must travel. If it be asked, not what is the velocity of electricity, but what is its time of transmission, in any particular case, there would be more hope of a definite answer. The distinction just indicated will do much towards reconciling the contradictory results of experiment in regard to what is erroneously called the velocity of electricity; these experiments making the velocity appear to be sometimes as great as 288,000 miles a second, and sometimes no more than 800 miles a second. In the first case the experiment was made on a very short conductor, and in the second case on a conductor of great length.

*See my Michigan Report, vol. i, pp. 126, 156.

From the Proceedings of the American Association for the Advancement of Science.

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