ON THE MOVEMENTS OF THE EARTH'S CRUST.

BY A. BLYTT.*

Translated by W. S. DALLAS, F. L. S.

This memoir is an attempt to further develop and establish ideas which I put forward five years ago. It.contains an attempt to establish a chronology in geology. It sets forth what the English call "a working hypothesis," without claiming to be anything else. It was the distribution of plants which first introduced the author to this great ques tion; but the problem of a chronology in geology can not be solved without the co-operation, it may perhaps be said, of all naturalists. It certainly can not finally be solved by any one man. In putting forth my hypothesis I must in the first place beg for indulgence for the many faults and imperfections with which such an attempt must be affected, and express a hope that in any case the hypothesis may be found worthy of being further tested.

Having endeavored in several memoirs on the distribution of plants, on peat-mosses, shore-lines, terraces, and morainic ridges, to show that climates undergo periodical changes, I published in the Transactions. of the Society of Sciences for 1883 (No. 9) a memoir on alternation of strata and its possible significance for the chronology of geology and the theory of the modification of species. The essential contents of this paper, as regards the present question of geological chronology, were as follows:

Alternations of strata, under which term is understood an alternation of geological formations of different constitution, can be produced by local conditions of rapidly passing change, without the action of general and persistent causes. But there are also causes of the latter kind which effect an alternation of the strata. Two such periodically acting causes are traceable in the geological series of deposits-a shorter,

*"On the probable cause of the Displacement of Shore-lines,-an attempt at a Geological Chronology." Read at the General Meetings of the Society of Science of Christiania, December 9, 1887, and June 1, 1888. Translated from the Nyt Magazin for Naturvidenskaberne, 1889; Bd. xxxi., pp, 240-297. (From the London, Edinburgh, and Dublin, Philosophical Magazine, May and June, 1889, vol. XXVII, pp. 405-429, and 487-519.)

somewhat regular one, and a longer, more irregular one. The former effects a change of climate, the strength of the marine currents alternately diminishing and increasing during thousands of years; the latter longer period effects a rise or fall of the sea in relation to the land, and an alternation of deep-sea formations with shore-formations or freshwater deposits. The opinion has been expressed that these periods, which are traced in the series of deposits, might possibly stand in connection with the two cosmical periods revealed by astronomy-the precession of the equinoctial points, and variations in the eccentricity of the earth's orbit; although in the memoir referred to it is not attempted to show in what manner such a connection could be established. But if, with the aid of these two hypotheses, we construct an "artificial" series of strata, we find that one with no less than forty-six changes of deposit may be recognized, bed by bed, in the Tertiary formations of the Paris basin.

The result may encourage us to test still further the correctness of the two suppositions. As regards the precession this has been attempted in my paper "On the probable cause of the periodical change in the strength of the marine currents."*

The contents of this memoir are essentially as follows: The precession of the equinoctial lines causes the summers in about 10,500 years to be longer, and in the following 10,500 years shorter, than the winters. The conditions are opposite in the northern and southern hemispheres. The difference between the number of winter and summer days increases with the eccentricity of the earth's orbit.

The cooling of continents under high latitudes, in the winter, produces a diminished pressure of air over the sea. This low pressure draws air from lower latitudes. For this reason, in the Atlantic, southwest winds prevail. Thus, in the winter, the southwest winds of the North Atlantic are on an average three times as strong as in the summer, in consequence of the great refrigeration of the mainland. In the semi-period when the winter falls in aphelion the average annual wind-force is consequently greater. Now it is the prevalent wind that produces the powerful marine currents, such as the warm current in the Atlantic Ocean. The strength of the marine currents is dependent upon the average wind-force for the last great time period. Now as this average wind-force is periodically variable in consequence of the precessions, the strength of marine currents and the temperature of the sea must also be subject to a periodical variability. For about 10,500 years the warm sea-current will increase, to diminish in the next similar period, and so on constantly through all time. When the winter falls in aphelion, the difference between the littoral and inland climates will increase. The propelling force of currents in the sea will increase and diminish by 1 to 5 per cent. upon their total annual value according as

* Vid. Selsk. Förh. Christiania, December 14, 1883; Archiv f. Math. og Naturv., IX. Christiania, 1884.

the winter falls in aphelion or perihelion, and according as the eccentricity of the earth's orbit is small or great.

Such an alteration in the strength of the marine currents will pro duce an alteration of the climate, which however will not be very im portant, but which will nevertheless be great enough to leave its traces in the deposits. During colder and drier seasons the streams are fed in great part by spring water. This water has drained slowly through the beds and is charged with dissolved materials; but the small quantity of water and the feebler streams carry less clay, sand, and gravel. During rainy seasons, the rain carries down quantities of such materials, but it flows off rapidly, and as it for the most part runs ouly over the surface it has not time to dissolve so much. Although the springs flow more abundantly during rainy seasons, their water only mingles with the rain-water. The streams are therefore poorer in dissolved material, but they contain more water, and their more powerful current carries more clay, sand, and gravel into the basin. Hence the drier seasons will be richer in purely chemical deposits, which will be transported in the clearer water; the wet seasons in mechanical deposits. Strata of both kinds are formed, of course, at all times, but they are deposited at different places in accordance with the variation in the quantity of rain. Thus, I assume that when thick deposits of riversand and clay alternate with each other, when soft clay and marl alternate with hard marl or limestone, when thick strata of loose sand alternate with sandstone, which is bound together by chemically produced cement (iron, silica, lime), when clay alternates with Septaria-beds, etc., then, in each case, the first-named deposit shows itself to belong to seasons with a warmer sea and a greater quantity of rain, which, as regards western Europe, will mean seasons with the winter in aphelion. That this alternation of deposits implies a period of several thousand years' duration is shown by the fact that the fossils change rapidly through the strata. In the Tertiary formations there are only a few, often only four to five, such changes of deposits in each stage. The whole Oligocene period has only about thirty, the Miocene still fewer, and the Pliocene barely twenty such changes.

In this way, in my opinion, the precessions stamp themselves upon the strata, and this should therefore furnish a means of measuring time. The greater the eccentricity of the orbit, the more strongly marked will the periods be; when the orbit approaches the circular form, they are less recognizable.* .

* But the perihelion also shifts to and fro. The time between two aphelia in the winter solstice varied thus in post-glacial times by fully 4600 years. This must have some influence. The longer a period with winters in aphelion lasts the longer will the warm currents in the Atlantic increase in strength, and the greater will be the changes of climate. The mild period during which Bergenian sea-animals lived in the Christiania Fjord, and which has left its traces elsewhere in our hemisphere, was in my opinion, a consequence of such an unusually long period with the winter in

Referring for other things to the two memoirs cited and to my paper "On Variations of the Weather in the course of time" (Letterstedtske Nordisk Tidskrift, 1885, in English, in Fork. Vid. Selsk. i Christiania, 1886, No. 8) I will pass on to examine whether there is any probable ground for supposing that the other proposition is also correct, whether it is conceivable that under high latitudes the sea-level rises and sinks with the eccentricity of the earth's orbit.

Great part of the earth's surface consists of strata which still lie undisturbed in their original horizontal position. These parts are called "tables" by Suess. But in many places the crust of the earth is so traversed by clefts and fissures that it may be compared to a breccia. Fragments are often displaced relatively by thousands of feet. Strata which originally lay horizontally are folded, thicknesses of 7,000 to 8,000 feet are bent as if they were straws (Kjerulf, Udsigt over Norges Geologi, 1879, p. 76). Moreover, the folded strata are upheaved far above their original level. Even marine formations so recent as the Eocene are uplifted to heights of 21,000 feet above the sea (Suess, Antlitz der Erde, I, p. 564). Sometimes they stand vertically, or are inverted, so that older strata cover the younger ones. Through fissures eruptive masses are brought forth, and have covered thousands upon thousands of square kilometers. The distribution of land and sea also varies. It is indeed supposed that the great depths of the ocean and the great continents have essentially retained their original distribution from the most ancient times, but the shore-lines wander periodically to and fro; and these changes of the earth's surface have taken place from earliest times, and are still in action at the present day.

Geologists in general seek the explanation of these phenomena in the cooling and contraction of the body of the earth. The earth's crust folds, just as the skin of an apple wrinkles as the apple dries. The leading geologists of the present day adopt this theory, and A. Geikie in his "Text-book of Geology" (London, 1882, p. 287) says truly: "With modifications, the main cause of terrestrial movements is still sought in secular contraction."

According to this doctrine changes in the crust of the earth are due to the interior contracting more strongly than "the crust," so that the latter is too large for it. Its weight drags it down. By this means great horizontally acting pressure is produced in the crust, which must then become folded and cracked in places. The fragments sink down. By this means are formed what Suess has called "Einbrüche." When a part of the crust remains in position while all around it sinks, there is produced what Suess has called a "Horst." The old theory of forces

aphelion. The winter solstice fell in aphelion (according to Croll) 61,300, 33,300, and 11,700 years ago. The middle of the Atlantic period with Bergenian sea-animals in the Christiania Fjord fell, from the testimony of the peat-mosses 33,000 to 34,000 years ago, therefore in accordance with the period of 28,000 years.