Some salient points in the science of the earth / by Sir J. William Dawson.
John W Dawson · Harper & Brothers eBooks · 1894
WORLD-MAKING1 1 themselves into systems of suns and planets, but with this difference from ours, that the suns were very large and sur- rounded with a wide luminous haze, and each of the planets was self-luminous, like a little sun.In some the planets were dancing up and down in spiral Hnes.In others they were moving in one plane.In still others, in every variety of direction.Some had vast numbers of little planets and satellites.Others had a few of larger size.There were even some of these systems that had a pair of central suns of con- trasting colours.The whole scene was so magnificent and beautiful that I thought I could never weary of gazing on it. "Here," said he, " we have the most beautiful condition of systems of worlds, when considered from a merely physical point of view : the perfection of solar and planetary luminousness, but which is destined to pass away in the interest of things more important, if less showy.This is the condition of the great star Sirius, which the old priest astronomers of the Nile Valley made so much of in their science and religion, and which they called Sothis.It is now known by your stargazers to be vastly larger than your sun, and fifty times more brilliant.^Letus select one of these systems somewhat similar to the solar system, and suppose that the luminous atmospheres of its nearer planets are beginning to wane in brilliancy.Here is one of them, through whose halo of light we can see the body of the planet.What do you now per- ceive ?"The planet referred to was somewhat larger in appear- ance than our earth, and, approaching near to it, I could see that it had a cloud-bearing firmament, and that it seemed to have continents and oceans, though disposed in more regular forms than on our own planet, and with a smaller proportion of land.Looking at it more closely, I searched in vain for ^In evidence of these and other statements I may refer to Huggins' recent address as President of the British Association, and to the " Story of the Heavens," etc., by Sir Robert Ball. WORLD-MAKINGany sign of animal life, but I saw a vast profusion of what might be plants, but not like those of this world.^These were trees of monstrous stature, and their leaves, which were of great size and shaped like fronds of seaweeds, were not usually green, but variegated with red, crimson and orange.The surface of the land looked like beds of gigantic specimens of Colias and similar variegated-leaved plants, the whole present- ing a most gorgeous yet grotesque spectacle. "This," said my guide, " is the primitive vegetation which clothes each of the planets in its youthful state.The earth was once so clothed, in the time when vegetable life alone existed, and there were no animals to prey upon it, and when the earth was, like the world you now look upon, a paradise of plants ; for all things in nature are at first in their best estate.This vegetation is known to you on the earth only by the Carbon and Graphite buried in your oldest rocks.It still lingers on your neighbour Mars,^w hich has, however, almost passed beyond this stage, and we are looking forward before long to see a still more gigantic though paler development of it in altogether novel shapes on the great continents that are being formed on the surface of Jupiter.But look again."And time being again annihilated, I saw the same world, now destitute of any luminous envelope, with a few dark clouds in its atmosphere, and presenting just the same appearance which I would sup- pose our earth to present to an astronomer viewing it with a powerful telescope from the moon." Here we are at home again," said my guide; "good-bye."I found myself nodding over my table, and that my pen had just dropped from my hand, making a large blot on my paper.My dream, however, * We shall see farther on that there is reason to believe that the primitive land vegetation was more different from that of the Devonian and Carboni- lerous than it is from that of the present day.'^M ars is probably a stage behind the earth in its development, and the ruddy hue of its continents would seem to bs due to some organic covering.•WORLD-MAKING 1 5 part of the history of the earth and of the system to which it belongs, nor does it enter at all into the more recondite problems involved ; still it forms, I believe, some necessary preparation at least to the comprehension of these.If we are to go farther back, we must accept the guidance of physicists rather than of geologists, and I must say that in this physical cosmology both geologists and general readers are likely to find themselves perplexed with the vagaries in which the most sober mathematicians may indulge.We are told that the original condition of the solar system was that of a vaporous and nebulous cloud intensely heated and whirling rapidly round, that it probably came into this condition by the impact of two dark solid bodies striking each other so violently, that they became intensely heated and resolved into the smallest possible fragments.Lord Kelvin attributes this impact to their being attracted together by gravitative force.Croll ârgues that in addition to gravitation these bodies must have had a proper motion of great velocity, which Lord Kelvin thinks " enormously " improbable, as it would require the solid bodies to be shot against each other with a marvellously true aim, and this not in the case of the sun only, but of all the stars.It is rather more improbable than it would be to affirm that in the artillery practice of two opposing armies, cannon balls have thousands of times struck and shattered each other midway between the hostile batteries.The ques- tion, we are told, is one of great moment to geologists, since on the one hypothesis the duration of our system has amounted to only about twenty millions of years ; on the other, it may have lasted ten times that number."In any case it seems a strange way of making systems of worlds, that they should result from the chance collision of multitudes of solid bodies 1 «* Stellar Evolution." 2 Other facts favour the shorter time (Clarence King, Am.Jl. of Sciencev ol.xlv., 3rd series).l6WORLD-MAKING rushing hither and thither in space, and it is almost equally strange to imagine an intelligent Creator banging these bodies about like billiard balls in order to make worlds.Still, in that case we might imagine them not to be altogether aimless.The question only becomes more complicated when with Grove and Lockyer we try to reach back to an antecedent condition, when there are neither solid masses nor nebulae, but only an inconceivably tenuous and universally diffused medium made up of an embryonic matter, which has not yet even resolved itself into chemical elements.How this could establish any motion within itself tending to aggregation in masses, is quite inconceivable.To ploddmg geologists laboriously collecting facts and framing conclusions therefrom, such flights of the mathematical mind seem like the wildest fantasies of dreams.We are glad to turn from them to examine those oldest rocks, which are to us the foundation stones of the earth's crust.What do we know of the oldest and most primitive rocks ?At this moment the question may be answered in many and discordant ways ; yet the leading elements of the answer may be given very simply.The oldest rock formation known to geologists is the Lower Laurentian, the Fundamental Gneiss, the Lewisian formation of Scotland, the Ottawa gneiss of Canada, the lowest Archaean crystalline rocks.This forma- tion, of enormous thickness, corresponds to what the older geologists called the fundamental granite, a name not to be scouted, for gneiss is only a stratified or laminated granite.Perhaps the main fact in relation to this old rock is that it is a gneiss ; that is, a rock at once bedded and crystalline, and having for its dominant ingredient the mineral orthoclase, a compound of silica, alumina and potash, in which are imbedded, as in a paste, grains and crystals of quartz and hornblende.We know very well from its texture and composition that it cannot be a product of mere heat, and being a bedded rock WORLD-MAKING crust than the material of our old gneisses.As to its bedded character, this may have arisen in part from the addition of cooling layers below, in part from the action of heated water above, and in part from pressure or tension ; while, wherever it cracked or became broken, its interstices would be injected with molten matter from beneath.All this may be conjecture, but it is based on known facts, and is the only probable con- jecture.If correct, it would account for the fact that the gneissic rocks are the lowest and oldest that we reach in every part of the earth.In short, the fundamental gneiss of the Lower Laurentian may have been the first rock ever formed ; and in any case it is a rock formed under conditions which have not since re- curred, except locally.It constitutes the first and best example of those chemico-physical, aqueous or aqueo-igneous rocks, so characteristic of the earliest period of the earth's history.Viewed in this way the Lower Laurentian gneiss is probably the oldest kind of rock we shall ever knowthe limit to our backward progress, beyond which there remains nothing to the geologist except physical hypotheses respecting a cooling incan- descent globe.For the chemical conditions of these primitive rocks, and what is known as to their probable origin, I may refer to the writings of my friends, the late Dr. Sterry Hunt and Dr. J. G. Bonney, to whom we owe so much of what is known of the older crystalline rocks^as well as of their literature, and the questions which they raise.My purpose here is to sketch the remarkable difference which we meet as we ascend into the Middle and Upper Laurentian.In the next succeeding formation, the middle part of the Laurentian of Logan, the Grenville series of Canada, we meet with a great and significant change.It is true we have still a predominance of gneisses which may have been formed in the 1 Hunt, "Essays on Chemical Geology"; Bonney,